Buggy python submission (verdict: Runtime Error):
import sys,bisect,string,math,time,functools,random,fractions
from heapq import heappush,heappop,heapify
from collections import deque,defaultdict,Counter
from itertools import permutations,combinations,groupby
rep=range
def Golf():n,*t=map(int,open(0).read().split())
def I():return int(input())
def S_():return input()
def IS():return input().split()
def LS():return [i for i in input().split()]
def MI():return map(int,input().split())
def LI():return [int(i) for i in input().split()]
def LI_():return [int(i)-1 for i in input().split()]
def NI(n):return [int(input()) for i in range(n)]
def NI_(n):return [int(input())-1 for i in range(n)]
def StoLI():return [ord(i)-97 for i in input()]
def ItoS(n):return chr(n+97)
def LtoS(ls):return ''.join([chr(i+97) for i in ls])
def RA():return map(int,open(0).read().split())
def GI(V,E,ls=None,Directed=False,index=1):
    org_inp=[];g=[[] for i in range(V)]
    FromStdin=True if ls==None else False
    for i in range(E):
        if FromStdin:
            inp=LI()
            org_inp.append(inp)
        else:
            inp=ls[i]
        if len(inp)==2:
            a,b=inp;c=1
        else:
            a,b,c=inp
        if index==1:a-=1;b-=1
        aa=(a,c);bb=(b,c);g[a].append(bb)
        if not Directed:g[b].append(aa)
    return g,org_inp
def GGI(h,w,search=None,replacement_of_found='.',mp_def={'#':1,'.':0},boundary=1):
    #h,w,g,sg=GGI(h,w,search=['S','G'],replacement_of_found='.',mp_def={'#':1,'.':0},boundary=1) # sample usage
    mp=[boundary]*(w+2);found={}
    for i in range(h):
        s=input()
        for char in search:
            if char in s:
                found[char]=((i+1)*(w+2)+s.index(char)+1)
                mp_def[char]=mp_def[replacement_of_found]
        mp+=[boundary]+[mp_def[j] for j in s]+[boundary]
    mp+=[boundary]*(w+2)
    return h+2,w+2,mp,found
def TI(n):return GI(n,n-1)
def accum(ls):
    rt=[0]
    for i in ls:rt+=[rt[-1]+i]
    return rt
def bit_combination(n,base=2):
    rt=[]
    for tb in range(base**n):s=[tb//(base**bt)%base for bt in range(n)];rt+=[s]
    return rt
def gcd(x,y):
    if y==0:return x
    if x%y==0:return y
    while x%y!=0:x,y=y,x%y
    return y
def YN(x):print(['NO','YES'][x])
def Yn(x):print(['No','Yes'][x])
def show(*inp,end='\n'):
    if show_flg:print(*inp,end=end)

mo=10**9+7
inf=float('inf')
FourNb=[(-1,0),(1,0),(0,1),(0,-1)];EightNb=[(-1,0),(1,0),(0,1),(0,-1),(1,1),(-1,-1),(1,-1),(-1,1)];compas=dict(zip('WENS',FourNb));cursol=dict(zip('LRUD',FourNb))
l_alp=string.ascii_lowercase
#sys.setrecursionlimit(10**7)
read=sys.stdin.buffer.read
readline=sys.stdin.buffer.readline
input=lambda: sys.stdin.readline().rstrip()

class Tree:
    def __init__(self,inp_size=None,ls=None,init=True,index=1):
        self.LCA_init_stat=False
        self.ETtable=[]
        if init:
            if ls==None:
                self.stdin(inp_size,index=index)
            else:
                self.node_size=len(ls)+1
                self.edges,_=GI(self.node_size,self.node_size-1,ls,index=index)
        return
 
    def stdin(self,inp_size=None,index=1):
        if inp_size==None:
            self.node_size=int(input())
        else:
            self.node_size=inp_size
        self.edges,_=GI(self.node_size,self.node_size-1,index=index)
        return
    
    def listin(self,ls,index=0):
        self.node_size=len(ls)+1
        self.edges,_=GI(self.node_size,self.node_size-1,ls,index=index)
        return
 
    def dfs(self,x,func=lambda pr,prv,nx,dist:prv+dist,root_v=0):
        q=deque([x])
        v=[None]*self.node_size
        v[x]=root_v
        while q:
            c=q.pop()
            for nb,d in self.edges[c]:
                if v[nb]==None:
                    q.append(nb)
                    v[nb]=func(c,v[c],nb,d)
        return v
 
    def bfs(self,x,func=lambda pr,prv,nx,dist:prv+dist,root_v=0):
        q=deque([x])
        v=[None]*self.node_size
        v[x]=root_v
        while q:
            c=q.popleft()
            for nb,d in self.edges[c]:
                if v[nb]==None:
                    q.append(nb)
                    v[nb]=func(c,v[c],nb,d)
        return v
 
    def parent(self,x):
        return self.dfs(0,func=lambda pr,prv,nx,dist:pr,root_v=-1)
 
    def topological_sort(self,x):  # return topological sort of the tree
        tps=[]
        q=deque([x])
        v=[None]*self.node_size
        v[x]=0
        while q:
            c=q.popleft()
            tps.append(c)
            for nb,d in self.edges[c]:
                if v[nb]==None:
                    q.append(nb)
                    v[nb]=0
        return tps
 
    def EulerTour(self,x):
        q=deque()
        q.append(x)
        self.depth=[None]*self.node_size
        self.depth[x]=0
        self.ETtable=[]
        self.ETdepth=[]
        self.ETin=[-1]*self.node_size
        self.ETout=[-1]*self.node_size
        cnt=0
        while q:
            c=q.pop()
            if c<0:
                ce=~c
            else:
                ce=c
                for nb,d in self.edges[ce]:
                    if self.depth[nb]==None:
                        q.append(~ce)
                        q.append(nb)
                        self.depth[nb]=self.depth[ce]+1
            self.ETtable.append(ce)
            self.ETdepth.append(self.depth[ce])
            if self.ETin[ce]==-1:
                self.ETin[ce]=cnt
            else:
                self.ETout[ce]=cnt
            cnt+=1
        return
    
    def LCA_init(self,root):
        self.EulerTour(root)
        self.st=SparseTable(self.ETdepth,init_func=min,init_idl=inf)
        #self.st=SegTree(self.node_size*2-1,self.ETdepth,function=min,ide=inf)
        self.LCA_init_stat=True
        return
    
    def LCA(self,root,x,y):
        if self.LCA_init_stat==False:
            self.LCA_init(root)
        xin,xout=self.ETin[x],self.ETout[x]
        yin,yout=self.ETin[y],self.ETout[y]
        a=min(xin,yin)
        b=max(xout,yout,xin,yin)
        id_of_min_dep_in_et=self.st.query_id(a,b+1)
        return self.ETtable[id_of_min_dep_in_et]
 
    def __str__(self):
        return  str(self.edges)
 
    def show(self):
        if all([all([d==1 for nd,d in e]) for e in self.edges]):
            print( [[nd for nd,d in e] for e in self.edges] )
        else:
            print(self)

class SparseTable: # O(N log N) for init, O(1) for query(l,r)
    def __init__(self,ls,init_func=min,init_idl=float('inf')):
        self.func=init_func
        self.idl=init_idl
        self.size=len(ls)
        self.N0=self.size.bit_length()
        self.table=[ls[:]]
        self.index=[list(range(self.size))]
        self.lg=[0]*(self.size+1)
        
        for i in range(2,self.size+1):
            self.lg[i]=self.lg[i>>1]+1
 
        for i in range(self.N0):
            tmp=[self.func(self.table[i][j],self.table[i][min(j+(1<<i),self.size-1)]) for j in range(self.size)]
            tmp_id=[self.index[i][j] if self.table[i][j]==self.func(self.table[i][j],self.table[i][min(j+(1<<i),self.size-1)]) else self.index[i][min(j+(1<<i),self.size-1)] for j in range(self.size)]
            self.table+=[tmp]
            self.index+=[tmp_id]
    
    # return func of [l,r)
    def query(self,l,r):
        if r>self.size:r=self.size
        #N=(r-l).bit_length()-1
        N=self.lg[r-l]
        return self.func(self.table[N][l],self.table[N][max(0,r-(1<<N))])
    
    # return index of which val[i] = func of v among [l,r)
    def query_id(self,l,r):
        if r>self.size:r=self.size
        #N=(r-l).bit_length()-1
        N=self.lg[r-l]
        a,b=self.index[N][l],self.index[N][max(0,r-(1<<N))]
        if self.table[0][a]==self.func(self.table[N][l],self.table[N][max(0,r-(1<<N))]):
            b=a
        return b
    
    # return boundary index of r such that func({table[i]} =< x , i in [pos,r]
    def right_bound(self,pos,x):
        k=(self.size-pos).bit_length()
        for j in range(k)[::-1]:
            nx=pos+(1<<j)
            if nx<n and self.query(pos,nx+1)<=x:
                pos+=(1<<j)
        return pos
    
    # return boundary index of l such that func({table[i]} =< x , i in [l,pos]
    def left_bound(self,pos,x):
        k=pos.bit_length()
        for j in range(k)[::-1]:
            nx=pos-(1<<j)
            if 0<=nx and self.query(nx,pos+1)<=x:
                pos-=(1<<j)
        return pos
 
    def __str__(self):
        return str(self.table[0])
 
    def print(self):
        for i in self.table:
            print(*i)


show_flg=False
show_flg=True
ans=0


class Graph:
    def __init__(self,inp_node_size=None,inp_edge_size=None,ls=None,init=True,Directed=False,index=1):
        if init:
            self.IsGrid=False
            if ls==None:
                self.stdin(inp_node_size,inp_edge_size,Directed=Directed,index=index)
            else:
                self.listin(ls,Directed=Directed,index=index)
        else:
            self.edges=[]
        return

    def stdin(self,inp_node_size=None,inp_edge_size=None,Directed=False,index=1):
        if (inp_node_size,inp_edge_size)==(None,None):
            self.node_size,self.edge_size=LI()
            self.edges,_=GI(self.node_size,self.edge_size,Directed=Directed,index=index)
        else:
            self.node_size,self.edge_size=inp_node_size,inp_edge_size
            self.edges,_=GI(self.node_size,self.edge_size,Directed=Directed,index=index)
        return

    def listin(self,inp_node_size,ls,Directed=False,index=0):
        self.node_size,self.edge_size=inp_node_size,len(ls)
        self.edges,_=GI(self.node_size,self.edge_size,ls,Directed=Directed,index=index)
        return

    def edgein(self,inp_edge,index=0):
        self.node_size,self.edge_size=len(inp_edge),0
        self.edges=[0]*self.node_size
        for i in range(self.node_size):
            self.edges[i]=inp_edge[i][:]
        return

    def gridin(self,h,w,search=None,replacement_of_found='.',mp_def={'#':1,'.':0}):
        #h,w,g,sg=GGI(h,w,search=['S','G'],replacement_of_found='.',mp_def={'#':1,'.':0},boundary=1) # sample usage
        self.mp,found=[],{}
        if search==None:
            search=[]
        self.node_size,self.edge_size,self.grid_h,self.grid_w=h*w,0,h,w
        self.edges=[[] for i in range(self.node_size)]
        self.IsGrid=True
        for i in range(h):
            s=input()
            for j in s:
                self.mp.append(mp_def[j])
        for dd in [1,w]:
            r_end=0
            for node in range(self.node_size):
                r_end+=1
                if r_end==w:
                    r_end=0
                nx=node+dd
                if nx<0 or self.node_size<=nx or (dd==1 and r_end==0):
                    continue
                if self.mp[nx]==0:
                    self.edges[node].append((nx,1))
                if self.mp[node]==0:
                    self.edges[nx].append((node,1))
        return found

    def dfs(self,x,func=lambda pr,prv,nx,dist:prv+dist,root_v=0):
        q=deque([x])
        v=[None]*self.node_size
        v[x]=root_v
        while q:
            c=q.pop()
            for nb,d in self.edges[c]:
                if v[nb]==None:
                    q.append(nb)
                    v[nb]=func(c,v[c],nb,d)
        return v

    def bfs(self,x,func=lambda pr,prv,nx,dist:prv+dist,root_v=0):
        q=deque([x])
        q=deque([i for i,j in enumerate(self.mp) if j==1])
        v=[None]*self.node_size
        #v[x]=root_v
        for y in q:v[y]=root_v
        while q:
            c=q.popleft()
            for nb,d in self.edges[c]:
                if v[nb]==None:
                    q.append(nb)
                    v[nb]=func(c,v[c],nb,d)
        return v

    def parent(self,x):
        return self.bfs(0,func=lambda pr,prv,nx,dist:pr,root_v=-1)

    def topological_sort(self,x):  # return topological sort of the tree
        tps=[]
        q=deque([x])
        v=[None]*self.node_size
        v[x]=0
        while q:
            c=q.popleft()
            tps.append(c)
            for nb,d in self.edges[c]:
                if v[nb]==None:
                    q.append(nb)
                    v[nb]=0
        return tps

    def dijkstra(self,st):
        # edge=[[(v_to,dist_to_v),...],[],...]
        # initialize: def: d=dist(st,i), prev=[previous vertex in minimum path], q[]
        d=[(0 if st==i else inf) for i in range(self.node_size)]
        prev=[0]*self.node_size
        q=[(j,i) for i,j in enumerate(d)]
        heapify(q)
        v=[False]*self.node_size
        # calc
        while q:
            dist,cur=heappop(q)
            if v[cur]:
                continue
            v[cur]=True
            for dst,dist in self.edges[cur]:
                alt=d[cur]+dist
                if alt<d[dst]:
                    d[dst]=alt
                    prev[dst]=cur
                    heappush(q,(alt,dst))
        return d,prev
    
    def __str__(self):
        if self.IsGrid:
            for i in range(self.grid_h):
                print(*self.mp[i*self.grid_w:-~i*self.grid_w])
            return str(str(self.show()))
        else:
            return  str(str(self.show()))

    def show(self):  # hide distance when all distance == 1
        if self.IsGrid:
            rt=[[nd for nd,d in e] for e in self.edges]
            return 'Grid',rt
        else:
            if all([all([d==1 for nd,d in e]) for e in self.edges]):
                rt=[[nd for nd,d in e] for e in self.edges]
                return 'Simplified',rt
            else:
                return self


gr=Graph(init=False)
h,w=LI()

gr.gridin(h,w)
print(0/0)
v=gr.bfs(0)

print(max(v))




Fixed submission:
from collections import deque
h,w=map(int,input().split())
a=[[] for _ in range(h)]
for i in range(h):
    s=input()
    for j in s:
        if j=='.':
            a[i].append(-1)
        else:
            a[i].append(0)

def bfs():
    d=[[0,1],[1,0],[-1,0],[0,-1]]
    q=deque()
    c=0
    for i in range(h):
        for j in range(w):
            if a[i][j]==0:
                q.append((i,j))
    while(len(q)>0):
        cur=q.popleft()
        i,j=cur
        for dd in d:
            di,dj=dd[0],dd[1]
            if (i+di)*(h-1-i-di)>=0 and (j+dj)*(w-1-j-dj)>=0 and a[i+di][j+dj]<0:
                c=a[i][j]+1
                a[i+di][j+dj]=c
                q.append((i+di,j+dj))
    return(c)

print(bfs())

Buggy python submission (verdict: Time Limit Exceeded):
import sys,bisect,string,math,time,functools,random,fractions
from heapq import heappush,heappop,heapify
from collections import deque,defaultdict,Counter
from itertools import permutations,combinations,groupby
rep=range
def Golf():n,*t=map(int,open(0).read().split())
def I():return int(input())
def S_():return input()
def IS():return input().split()
def LS():return [i for i in input().split()]
def MI():return map(int,input().split())
def LI():return [int(i) for i in input().split()]
def LI_():return [int(i)-1 for i in input().split()]
def NI(n):return [int(input()) for i in range(n)]
def NI_(n):return [int(input())-1 for i in range(n)]
def StoLI():return [ord(i)-97 for i in input()]
def ItoS(n):return chr(n+97)
def LtoS(ls):return ''.join([chr(i+97) for i in ls])
def RA():return map(int,open(0).read().split())
def GI(V,E,ls=None,Directed=False,index=1):
    org_inp=[];g=[[] for i in range(V)]
    FromStdin=True if ls==None else False
    for i in range(E):
        if FromStdin:
            inp=LI()
            org_inp.append(inp)
        else:
            inp=ls[i]
        if len(inp)==2:
            a,b=inp;c=1
        else:
            a,b,c=inp
        if index==1:a-=1;b-=1
        aa=(a,c);bb=(b,c);g[a].append(bb)
        if not Directed:g[b].append(aa)
    return g,org_inp
def GGI(h,w,search=None,replacement_of_found='.',mp_def={'#':1,'.':0},boundary=1):
    #h,w,g,sg=GGI(h,w,search=['S','G'],replacement_of_found='.',mp_def={'#':1,'.':0},boundary=1) # sample usage
    mp=[boundary]*(w+2);found={}
    for i in range(h):
        s=input()
        for char in search:
            if char in s:
                found[char]=((i+1)*(w+2)+s.index(char)+1)
                mp_def[char]=mp_def[replacement_of_found]
        mp+=[boundary]+[mp_def[j] for j in s]+[boundary]
    mp+=[boundary]*(w+2)
    return h+2,w+2,mp,found
def TI(n):return GI(n,n-1)
def accum(ls):
    rt=[0]
    for i in ls:rt+=[rt[-1]+i]
    return rt
def bit_combination(n,base=2):
    rt=[]
    for tb in range(base**n):s=[tb//(base**bt)%base for bt in range(n)];rt+=[s]
    return rt
def gcd(x,y):
    if y==0:return x
    if x%y==0:return y
    while x%y!=0:x,y=y,x%y
    return y
def YN(x):print(['NO','YES'][x])
def Yn(x):print(['No','Yes'][x])
def show(*inp,end='\n'):
    if show_flg:print(*inp,end=end)

mo=10**9+7
inf=float('inf')
FourNb=[(-1,0),(1,0),(0,1),(0,-1)];EightNb=[(-1,0),(1,0),(0,1),(0,-1),(1,1),(-1,-1),(1,-1),(-1,1)];compas=dict(zip('WENS',FourNb));cursol=dict(zip('LRUD',FourNb))
l_alp=string.ascii_lowercase
#sys.setrecursionlimit(10**7)
read=sys.stdin.buffer.read
readline=sys.stdin.buffer.readline
input=lambda: sys.stdin.readline().rstrip()

class Tree:
    def __init__(self,inp_size=None,ls=None,init=True,index=1):
        self.LCA_init_stat=False
        self.ETtable=[]
        if init:
            if ls==None:
                self.stdin(inp_size,index=index)
            else:
                self.node_size=len(ls)+1
                self.edges,_=GI(self.node_size,self.node_size-1,ls,index=index)
        return
 
    def stdin(self,inp_size=None,index=1):
        if inp_size==None:
            self.node_size=int(input())
        else:
            self.node_size=inp_size
        self.edges,_=GI(self.node_size,self.node_size-1,index=index)
        return
    
    def listin(self,ls,index=0):
        self.node_size=len(ls)+1
        self.edges,_=GI(self.node_size,self.node_size-1,ls,index=index)
        return
 
    def dfs(self,x,func=lambda pr,prv,nx,dist:prv+dist,root_v=0):
        q=deque([x])
        v=[None]*self.node_size
        v[x]=root_v
        while q:
            c=q.pop()
            for nb,d in self.edges[c]:
                if v[nb]==None:
                    q.append(nb)
                    v[nb]=func(c,v[c],nb,d)
        return v
 
    def bfs(self,x,func=lambda pr,prv,nx,dist:prv+dist,root_v=0):
        q=deque([x])
        v=[None]*self.node_size
        v[x]=root_v
        while q:
            c=q.popleft()
            for nb,d in self.edges[c]:
                if v[nb]==None:
                    q.append(nb)
                    v[nb]=func(c,v[c],nb,d)
        return v
 
    def parent(self,x):
        return self.dfs(0,func=lambda pr,prv,nx,dist:pr,root_v=-1)
 
    def topological_sort(self,x):  # return topological sort of the tree
        tps=[]
        q=deque([x])
        v=[None]*self.node_size
        v[x]=0
        while q:
            c=q.popleft()
            tps.append(c)
            for nb,d in self.edges[c]:
                if v[nb]==None:
                    q.append(nb)
                    v[nb]=0
        return tps
 
    def EulerTour(self,x):
        q=deque()
        q.append(x)
        self.depth=[None]*self.node_size
        self.depth[x]=0
        self.ETtable=[]
        self.ETdepth=[]
        self.ETin=[-1]*self.node_size
        self.ETout=[-1]*self.node_size
        cnt=0
        while q:
            c=q.pop()
            if c<0:
                ce=~c
            else:
                ce=c
                for nb,d in self.edges[ce]:
                    if self.depth[nb]==None:
                        q.append(~ce)
                        q.append(nb)
                        self.depth[nb]=self.depth[ce]+1
            self.ETtable.append(ce)
            self.ETdepth.append(self.depth[ce])
            if self.ETin[ce]==-1:
                self.ETin[ce]=cnt
            else:
                self.ETout[ce]=cnt
            cnt+=1
        return
    
    def LCA_init(self,root):
        self.EulerTour(root)
        self.st=SparseTable(self.ETdepth,init_func=min,init_idl=inf)
        #self.st=SegTree(self.node_size*2-1,self.ETdepth,function=min,ide=inf)
        self.LCA_init_stat=True
        return
    
    def LCA(self,root,x,y):
        if self.LCA_init_stat==False:
            self.LCA_init(root)
        xin,xout=self.ETin[x],self.ETout[x]
        yin,yout=self.ETin[y],self.ETout[y]
        a=min(xin,yin)
        b=max(xout,yout,xin,yin)
        id_of_min_dep_in_et=self.st.query_id(a,b+1)
        return self.ETtable[id_of_min_dep_in_et]
 
    def __str__(self):
        return  str(self.edges)
 
    def show(self):
        if all([all([d==1 for nd,d in e]) for e in self.edges]):
            print( [[nd for nd,d in e] for e in self.edges] )
        else:
            print(self)

class SparseTable: # O(N log N) for init, O(1) for query(l,r)
    def __init__(self,ls,init_func=min,init_idl=float('inf')):
        self.func=init_func
        self.idl=init_idl
        self.size=len(ls)
        self.N0=self.size.bit_length()
        self.table=[ls[:]]
        self.index=[list(range(self.size))]
        self.lg=[0]*(self.size+1)
        
        for i in range(2,self.size+1):
            self.lg[i]=self.lg[i>>1]+1
 
        for i in range(self.N0):
            tmp=[self.func(self.table[i][j],self.table[i][min(j+(1<<i),self.size-1)]) for j in range(self.size)]
            tmp_id=[self.index[i][j] if self.table[i][j]==self.func(self.table[i][j],self.table[i][min(j+(1<<i),self.size-1)]) else self.index[i][min(j+(1<<i),self.size-1)] for j in range(self.size)]
            self.table+=[tmp]
            self.index+=[tmp_id]
    
    # return func of [l,r)
    def query(self,l,r):
        if r>self.size:r=self.size
        #N=(r-l).bit_length()-1
        N=self.lg[r-l]
        return self.func(self.table[N][l],self.table[N][max(0,r-(1<<N))])
    
    # return index of which val[i] = func of v among [l,r)
    def query_id(self,l,r):
        if r>self.size:r=self.size
        #N=(r-l).bit_length()-1
        N=self.lg[r-l]
        a,b=self.index[N][l],self.index[N][max(0,r-(1<<N))]
        if self.table[0][a]==self.func(self.table[N][l],self.table[N][max(0,r-(1<<N))]):
            b=a
        return b
    
    # return boundary index of r such that func({table[i]} =< x , i in [pos,r]
    def right_bound(self,pos,x):
        k=(self.size-pos).bit_length()
        for j in range(k)[::-1]:
            nx=pos+(1<<j)
            if nx<n and self.query(pos,nx+1)<=x:
                pos+=(1<<j)
        return pos
    
    # return boundary index of l such that func({table[i]} =< x , i in [l,pos]
    def left_bound(self,pos,x):
        k=pos.bit_length()
        for j in range(k)[::-1]:
            nx=pos-(1<<j)
            if 0<=nx and self.query(nx,pos+1)<=x:
                pos-=(1<<j)
        return pos
 
    def __str__(self):
        return str(self.table[0])
 
    def print(self):
        for i in self.table:
            print(*i)


show_flg=False
show_flg=True
ans=0


class Graph:
    def __init__(self,inp_node_size=None,inp_edge_size=None,ls=None,init=True,Directed=False,index=1):
        if init:
            self.IsGrid=False
            if ls==None:
                self.stdin(inp_node_size,inp_edge_size,Directed=Directed,index=index)
            else:
                self.listin(ls,Directed=Directed,index=index)
        else:
            self.edges=[]
        return

    def stdin(self,inp_node_size=None,inp_edge_size=None,Directed=False,index=1):
        if (inp_node_size,inp_edge_size)==(None,None):
            self.node_size,self.edge_size=LI()
            self.edges,_=GI(self.node_size,self.edge_size,Directed=Directed,index=index)
        else:
            self.node_size,self.edge_size=inp_node_size,inp_edge_size
            self.edges,_=GI(self.node_size,self.edge_size,Directed=Directed,index=index)
        return

    def listin(self,inp_node_size,ls,Directed=False,index=0):
        self.node_size,self.edge_size=inp_node_size,len(ls)
        self.edges,_=GI(self.node_size,self.edge_size,ls,Directed=Directed,index=index)
        return

    def edgein(self,inp_edge,index=0):
        self.node_size,self.edge_size=len(inp_edge),0
        self.edges=[0]*self.node_size
        for i in range(self.node_size):
            self.edges[i]=inp_edge[i][:]
        return

    def gridin(self,h,w,search=None,replacement_of_found='.',mp_def={'#':1,'.':0}):
        #h,w,g,sg=GGI(h,w,search=['S','G'],replacement_of_found='.',mp_def={'#':1,'.':0},boundary=1) # sample usage
        self.mp,found=[],{}
        if search==None:
            search=[]
        self.node_size,self.edge_size,self.grid_h,self.grid_w=h*w,0,h,w
        self.edges=[[] for i in range(self.node_size)]
        self.IsGrid=True
        for i in range(h):
            s=input()
            for char in search:
                if char in s:
                    found[char]=(i*w+s.index(char))
                    mp_def[char]=mp_def[replacement_of_found]
            for j in s:
                self.mp.append(mp_def[j])
        for dd in [1,w]:
            r_end=0
            for node in range(self.node_size):
                r_end+=1
                if r_end==w:
                    r_end=0
                nx=node+dd
                if nx<0 or self.node_size<=nx or (dd==1 and r_end==0):
                    continue
                if self.mp[nx]==0:
                    self.edges[node].append((nx,1))
                if self.mp[node]==0:
                    self.edges[nx].append((node,1))
        return found

    def dfs(self,x,func=lambda pr,prv,nx,dist:prv+dist,root_v=0):
        q=deque([x])
        v=[None]*self.node_size
        v[x]=root_v
        while q:
            c=q.pop()
            for nb,d in self.edges[c]:
                if v[nb]==None:
                    q.append(nb)
                    v[nb]=func(c,v[c],nb,d)
        return v

    def bfs(self,x,func=lambda pr,prv,nx,dist:prv+dist,root_v=0):
        q=deque([x])
        q=deque([i for i,j in enumerate(self.mp) if j==1])
        v=[None]*self.node_size
        #v[x]=root_v
        for y in q:v[y]=root_v
        while q:
            c=q.popleft()
            for nb,d in self.edges[c]:
                if v[nb]==None:
                    q.append(nb)
                    v[nb]=func(c,v[c],nb,d)
        return v

    def parent(self,x):
        return self.bfs(0,func=lambda pr,prv,nx,dist:pr,root_v=-1)

    def topological_sort(self,x):  # return topological sort of the tree
        tps=[]
        q=deque([x])
        v=[None]*self.node_size
        v[x]=0
        while q:
            c=q.popleft()
            tps.append(c)
            for nb,d in self.edges[c]:
                if v[nb]==None:
                    q.append(nb)
                    v[nb]=0
        return tps

    def dijkstra(self,st):
        # edge=[[(v_to,dist_to_v),...],[],...]
        # initialize: def: d=dist(st,i), prev=[previous vertex in minimum path], q[]
        d=[(0 if st==i else inf) for i in range(self.node_size)]
        prev=[0]*self.node_size
        q=[(j,i) for i,j in enumerate(d)]
        heapify(q)
        v=[False]*self.node_size
        # calc
        while q:
            dist,cur=heappop(q)
            if v[cur]:
                continue
            v[cur]=True
            for dst,dist in self.edges[cur]:
                alt=d[cur]+dist
                if alt<d[dst]:
                    d[dst]=alt
                    prev[dst]=cur
                    heappush(q,(alt,dst))
        return d,prev
    
    def __str__(self):
        if self.IsGrid:
            for i in range(self.grid_h):
                print(*self.mp[i*self.grid_w:-~i*self.grid_w])
            return str(str(self.show()))
        else:
            return  str(str(self.show()))

    def show(self):  # hide distance when all distance == 1
        if self.IsGrid:
            rt=[[nd for nd,d in e] for e in self.edges]
            return 'Grid',rt
        else:
            if all([all([d==1 for nd,d in e]) for e in self.edges]):
                rt=[[nd for nd,d in e] for e in self.edges]
                return 'Simplified',rt
            else:
                return self


gr=Graph(init=False)
h,w=LI()

f=gr.gridin(h,w)
v=gr.bfs(0)

print(max(v))




Fixed submission:
from collections import deque
h,w=map(int,input().split())
a=[[] for _ in range(h)]
for i in range(h):
    s=input()
    for j in s:
        if j=='.':
            a[i].append(-1)
        else:
            a[i].append(0)

def bfs():
    d=[[0,1],[1,0],[-1,0],[0,-1]]
    q=deque()
    c=0
    for i in range(h):
        for j in range(w):
            if a[i][j]==0:
                q.append((i,j))
    while(len(q)>0):
        cur=q.popleft()
        i,j=cur
        for dd in d:
            di,dj=dd[0],dd[1]
            if (i+di)*(h-1-i-di)>=0 and (j+dj)*(w-1-j-dj)>=0 and a[i+di][j+dj]<0:
                c=a[i][j]+1
                a[i+di][j+dj]=c
                q.append((i+di,j+dj))
    return(c)

print(bfs())

Buggy python submission (verdict: Runtime Error):
import sys,bisect,string,math,time,functools,random,fractions
from heapq import heappush,heappop,heapify
from collections import deque,defaultdict,Counter
from itertools import permutations,combinations,groupby
rep=range
def Golf():n,*t=map(int,open(0).read().split())
def I():return int(input())
def S_():return input()
def IS():return input().split()
def LS():return [i for i in input().split()]
def MI():return map(int,input().split())
def LI():return [int(i) for i in input().split()]
def LI_():return [int(i)-1 for i in input().split()]
def NI(n):return [int(input()) for i in range(n)]
def NI_(n):return [int(input())-1 for i in range(n)]
def StoLI():return [ord(i)-97 for i in input()]
def ItoS(n):return chr(n+97)
def LtoS(ls):return ''.join([chr(i+97) for i in ls])
def RA():return map(int,open(0).read().split())
def GI(V,E,ls=None,Directed=False,index=1):
    org_inp=[];g=[[] for i in range(V)]
    FromStdin=True if ls==None else False
    for i in range(E):
        if FromStdin:
            inp=LI()
            org_inp.append(inp)
        else:
            inp=ls[i]
        if len(inp)==2:
            a,b=inp;c=1
        else:
            a,b,c=inp
        if index==1:a-=1;b-=1
        aa=(a,c);bb=(b,c);g[a].append(bb)
        if not Directed:g[b].append(aa)
    return g,org_inp
def GGI(h,w,search=None,replacement_of_found='.',mp_def={'#':1,'.':0},boundary=1):
    #h,w,g,sg=GGI(h,w,search=['S','G'],replacement_of_found='.',mp_def={'#':1,'.':0},boundary=1) # sample usage
    mp=[boundary]*(w+2);found={}
    for i in range(h):
        s=input()
        for char in search:
            if char in s:
                found[char]=((i+1)*(w+2)+s.index(char)+1)
                mp_def[char]=mp_def[replacement_of_found]
        mp+=[boundary]+[mp_def[j] for j in s]+[boundary]
    mp+=[boundary]*(w+2)
    return h+2,w+2,mp,found
def TI(n):return GI(n,n-1)
def accum(ls):
    rt=[0]
    for i in ls:rt+=[rt[-1]+i]
    return rt
def bit_combination(n,base=2):
    rt=[]
    for tb in range(base**n):s=[tb//(base**bt)%base for bt in range(n)];rt+=[s]
    return rt
def gcd(x,y):
    if y==0:return x
    if x%y==0:return y
    while x%y!=0:x,y=y,x%y
    return y
def YN(x):print(['NO','YES'][x])
def Yn(x):print(['No','Yes'][x])
def show(*inp,end='\n'):
    if show_flg:print(*inp,end=end)

mo=10**9+7
inf=float('inf')
FourNb=[(-1,0),(1,0),(0,1),(0,-1)];EightNb=[(-1,0),(1,0),(0,1),(0,-1),(1,1),(-1,-1),(1,-1),(-1,1)];compas=dict(zip('WENS',FourNb));cursol=dict(zip('LRUD',FourNb))
l_alp=string.ascii_lowercase
#sys.setrecursionlimit(10**7)
read=sys.stdin.buffer.read
readline=sys.stdin.buffer.readline
input=lambda: sys.stdin.readline().rstrip()

class Tree:
    def __init__(self,inp_size=None,ls=None,init=True,index=1):
        self.LCA_init_stat=False
        self.ETtable=[]
        if init:
            if ls==None:
                self.stdin(inp_size,index=index)
            else:
                self.node_size=len(ls)+1
                self.edges,_=GI(self.node_size,self.node_size-1,ls,index=index)
        return
 
    def stdin(self,inp_size=None,index=1):
        if inp_size==None:
            self.node_size=int(input())
        else:
            self.node_size=inp_size
        self.edges,_=GI(self.node_size,self.node_size-1,index=index)
        return
    
    def listin(self,ls,index=0):
        self.node_size=len(ls)+1
        self.edges,_=GI(self.node_size,self.node_size-1,ls,index=index)
        return
 
    def dfs(self,x,func=lambda pr,prv,nx,dist:prv+dist,root_v=0):
        q=deque([x])
        v=[None]*self.node_size
        v[x]=root_v
        while q:
            c=q.pop()
            for nb,d in self.edges[c]:
                if v[nb]==None:
                    q.append(nb)
                    v[nb]=func(c,v[c],nb,d)
        return v
 
    def bfs(self,x,func=lambda pr,prv,nx,dist:prv+dist,root_v=0):
        q=deque([x])
        v=[None]*self.node_size
        v[x]=root_v
        while q:
            c=q.popleft()
            for nb,d in self.edges[c]:
                if v[nb]==None:
                    q.append(nb)
                    v[nb]=func(c,v[c],nb,d)
        return v
 
    def parent(self,x):
        return self.dfs(0,func=lambda pr,prv,nx,dist:pr,root_v=-1)
 
    def topological_sort(self,x):  # return topological sort of the tree
        tps=[]
        q=deque([x])
        v=[None]*self.node_size
        v[x]=0
        while q:
            c=q.popleft()
            tps.append(c)
            for nb,d in self.edges[c]:
                if v[nb]==None:
                    q.append(nb)
                    v[nb]=0
        return tps
 
    def EulerTour(self,x):
        q=deque()
        q.append(x)
        self.depth=[None]*self.node_size
        self.depth[x]=0
        self.ETtable=[]
        self.ETdepth=[]
        self.ETin=[-1]*self.node_size
        self.ETout=[-1]*self.node_size
        cnt=0
        while q:
            c=q.pop()
            if c<0:
                ce=~c
            else:
                ce=c
                for nb,d in self.edges[ce]:
                    if self.depth[nb]==None:
                        q.append(~ce)
                        q.append(nb)
                        self.depth[nb]=self.depth[ce]+1
            self.ETtable.append(ce)
            self.ETdepth.append(self.depth[ce])
            if self.ETin[ce]==-1:
                self.ETin[ce]=cnt
            else:
                self.ETout[ce]=cnt
            cnt+=1
        return
    
    def LCA_init(self,root):
        self.EulerTour(root)
        self.st=SparseTable(self.ETdepth,init_func=min,init_idl=inf)
        #self.st=SegTree(self.node_size*2-1,self.ETdepth,function=min,ide=inf)
        self.LCA_init_stat=True
        return
    
    def LCA(self,root,x,y):
        if self.LCA_init_stat==False:
            self.LCA_init(root)
        xin,xout=self.ETin[x],self.ETout[x]
        yin,yout=self.ETin[y],self.ETout[y]
        a=min(xin,yin)
        b=max(xout,yout,xin,yin)
        id_of_min_dep_in_et=self.st.query_id(a,b+1)
        return self.ETtable[id_of_min_dep_in_et]
 
    def __str__(self):
        return  str(self.edges)
 
    def show(self):
        if all([all([d==1 for nd,d in e]) for e in self.edges]):
            print( [[nd for nd,d in e] for e in self.edges] )
        else:
            print(self)

class SparseTable: # O(N log N) for init, O(1) for query(l,r)
    def __init__(self,ls,init_func=min,init_idl=float('inf')):
        self.func=init_func
        self.idl=init_idl
        self.size=len(ls)
        self.N0=self.size.bit_length()
        self.table=[ls[:]]
        self.index=[list(range(self.size))]
        self.lg=[0]*(self.size+1)
        
        for i in range(2,self.size+1):
            self.lg[i]=self.lg[i>>1]+1
 
        for i in range(self.N0):
            tmp=[self.func(self.table[i][j],self.table[i][min(j+(1<<i),self.size-1)]) for j in range(self.size)]
            tmp_id=[self.index[i][j] if self.table[i][j]==self.func(self.table[i][j],self.table[i][min(j+(1<<i),self.size-1)]) else self.index[i][min(j+(1<<i),self.size-1)] for j in range(self.size)]
            self.table+=[tmp]
            self.index+=[tmp_id]
    
    # return func of [l,r)
    def query(self,l,r):
        if r>self.size:r=self.size
        #N=(r-l).bit_length()-1
        N=self.lg[r-l]
        return self.func(self.table[N][l],self.table[N][max(0,r-(1<<N))])
    
    # return index of which val[i] = func of v among [l,r)
    def query_id(self,l,r):
        if r>self.size:r=self.size
        #N=(r-l).bit_length()-1
        N=self.lg[r-l]
        a,b=self.index[N][l],self.index[N][max(0,r-(1<<N))]
        if self.table[0][a]==self.func(self.table[N][l],self.table[N][max(0,r-(1<<N))]):
            b=a
        return b
    
    # return boundary index of r such that func({table[i]} =< x , i in [pos,r]
    def right_bound(self,pos,x):
        k=(self.size-pos).bit_length()
        for j in range(k)[::-1]:
            nx=pos+(1<<j)
            if nx<n and self.query(pos,nx+1)<=x:
                pos+=(1<<j)
        return pos
    
    # return boundary index of l such that func({table[i]} =< x , i in [l,pos]
    def left_bound(self,pos,x):
        k=pos.bit_length()
        for j in range(k)[::-1]:
            nx=pos-(1<<j)
            if 0<=nx and self.query(nx,pos+1)<=x:
                pos-=(1<<j)
        return pos
 
    def __str__(self):
        return str(self.table[0])
 
    def print(self):
        for i in self.table:
            print(*i)


show_flg=False
show_flg=True
ans=0


class Graph:
    def __init__(self,inp_node_size=None,inp_edge_size=None,ls=None,init=True,Directed=False,index=1):
        if init:
            self.IsGrid=False
            if ls==None:
                self.stdin(inp_node_size,inp_edge_size,Directed=Directed,index=index)
            else:
                self.listin(ls,Directed=Directed,index=index)
        else:
            self.edges=[]
        return

    def stdin(self,inp_node_size=None,inp_edge_size=None,Directed=False,index=1):
        if (inp_node_size,inp_edge_size)==(None,None):
            self.node_size,self.edge_size=LI()
            self.edges,_=GI(self.node_size,self.edge_size,Directed=Directed,index=index)
        else:
            self.node_size,self.edge_size=inp_node_size,inp_edge_size
            self.edges,_=GI(self.node_size,self.edge_size,Directed=Directed,index=index)
        return

    def listin(self,inp_node_size,ls,Directed=False,index=0):
        self.node_size,self.edge_size=inp_node_size,len(ls)
        self.edges,_=GI(self.node_size,self.edge_size,ls,Directed=Directed,index=index)
        return

    def edgein(self,inp_edge,index=0):
        self.node_size,self.edge_size=len(inp_edge),0
        self.edges=[0]*self.node_size
        for i in range(self.node_size):
            self.edges[i]=inp_edge[i][:]
        return

    def gridin(self,h,w,search=None,replacement_of_found='.',mp_def={'#':1,'.':0}):
        #h,w,g,sg=GGI(h,w,search=['S','G'],replacement_of_found='.',mp_def={'#':1,'.':0},boundary=1) # sample usage
        self.mp,found=[],{}
        if search==None:
            search=[]
        self.node_size,self.edge_size,self.grid_h,self.grid_w=h*w,0,h,w
        self.edges=[[] for i in range(self.node_size)]
        self.IsGrid=True
        for i in range(h):
            s=input()
            self.mp+=[mp_def[j] for j in s]
        for dd in [1,w]:
            for node in range(self.node_size):
                nx=node+dd
                if nx<0 or self.node_size<=nx or (dd==1 and nx%w==0):
                    continue
                if self.mp[nx]==0:
                    self.edges[node].append((nx,1))
                if self.mp[node]==0:
                    self.edges[nx].append((node,1))
        return found

    def dfs(self,x,func=lambda pr,prv,nx,dist:prv+dist,root_v=0):
        q=deque([x])
        v=[None]*self.node_size
        v[x]=root_v
        while q:
            c=q.pop()
            for nb,d in self.edges[c]:
                if v[nb]==None:
                    q.append(nb)
                    v[nb]=func(c,v[c],nb,d)
        return v

    def bfs(self,x,func=lambda pr,prv,nx,dist:prv+dist,root_v=0):
        q=deque([x])
        q=deque([i for i,j in enumerate(self.mp) if j==1])
        v=[None]*self.node_size
        #v[x]=root_v
        for y in q:v[y]=root_v
        while q:
            c=q.popleft()
            for nb,d in self.edges[c]:
                if v[nb]==None:
                    q.append(nb)
                    v[nb]=func(c,v[c],nb,d)
        return v

    def parent(self,x):
        return self.bfs(0,func=lambda pr,prv,nx,dist:pr,root_v=-1)

    def topological_sort(self,x):  # return topological sort of the tree
        tps=[]
        q=deque([x])
        v=[None]*self.node_size
        v[x]=0
        while q:
            c=q.popleft()
            tps.append(c)
            for nb,d in self.edges[c]:
                if v[nb]==None:
                    q.append(nb)
                    v[nb]=0
        return tps

    def dijkstra(self,st):
        # edge=[[(v_to,dist_to_v),...],[],...]
        # initialize: def: d=dist(st,i), prev=[previous vertex in minimum path], q[]
        d=[(0 if st==i else inf) for i in range(self.node_size)]
        prev=[0]*self.node_size
        q=[(j,i) for i,j in enumerate(d)]
        heapify(q)
        v=[False]*self.node_size
        # calc
        while q:
            dist,cur=heappop(q)
            if v[cur]:
                continue
            v[cur]=True
            for dst,dist in self.edges[cur]:
                alt=d[cur]+dist
                if alt<d[dst]:
                    d[dst]=alt
                    prev[dst]=cur
                    heappush(q,(alt,dst))
        return d,prev
    
    def __str__(self):
        if self.IsGrid:
            for i in range(self.grid_h):
                print(*self.mp[i*self.grid_w:-~i*self.grid_w])
            return str(str(self.show()))
        else:
            return  str(str(self.show()))

    def show(self):  # hide distance when all distance == 1
        if self.IsGrid:
            rt=[[nd for nd,d in e] for e in self.edges]
            return 'Grid',rt
        else:
            if all([all([d==1 for nd,d in e]) for e in self.edges]):
                rt=[[nd for nd,d in e] for e in self.edges]
                return 'Simplified',rt
            else:
                return self


gr=Graph(init=False)
h,w=LI()

gr.gridin(h,w)
print(0/0)
v=gr.bfs(0)

print(max(v))




Fixed submission:
from collections import deque
h,w=map(int,input().split())
a=[[] for _ in range(h)]
for i in range(h):
    s=input()
    for j in s:
        if j=='.':
            a[i].append(-1)
        else:
            a[i].append(0)

def bfs():
    d=[[0,1],[1,0],[-1,0],[0,-1]]
    q=deque()
    c=0
    for i in range(h):
        for j in range(w):
            if a[i][j]==0:
                q.append((i,j))
    while(len(q)>0):
        cur=q.popleft()
        i,j=cur
        for dd in d:
            di,dj=dd[0],dd[1]
            if (i+di)*(h-1-i-di)>=0 and (j+dj)*(w-1-j-dj)>=0 and a[i+di][j+dj]<0:
                c=a[i][j]+1
                a[i+di][j+dj]=c
                q.append((i+di,j+dj))
    return(c)

print(bfs())

Buggy python submission (verdict: Time Limit Exceeded):
import sys,bisect,string,math,time,functools,random,fractions
from heapq import heappush,heappop,heapify
from collections import deque,defaultdict,Counter
from itertools import permutations,combinations,groupby
rep=range
def Golf():n,*t=map(int,open(0).read().split())
def I():return int(input())
def S_():return input()
def IS():return input().split()
def LS():return [i for i in input().split()]
def MI():return map(int,input().split())
def LI():return [int(i) for i in input().split()]
def LI_():return [int(i)-1 for i in input().split()]
def NI(n):return [int(input()) for i in range(n)]
def NI_(n):return [int(input())-1 for i in range(n)]
def StoLI():return [ord(i)-97 for i in input()]
def ItoS(n):return chr(n+97)
def LtoS(ls):return ''.join([chr(i+97) for i in ls])
def RA():return map(int,open(0).read().split())
def GI(V,E,ls=None,Directed=False,index=1):
    org_inp=[];g=[[] for i in range(V)]
    FromStdin=True if ls==None else False
    for i in range(E):
        if FromStdin:
            inp=LI()
            org_inp.append(inp)
        else:
            inp=ls[i]
        if len(inp)==2:
            a,b=inp;c=1
        else:
            a,b,c=inp
        if index==1:a-=1;b-=1
        aa=(a,c);bb=(b,c);g[a].append(bb)
        if not Directed:g[b].append(aa)
    return g,org_inp
def GGI(h,w,search=None,replacement_of_found='.',mp_def={'#':1,'.':0},boundary=1):
    #h,w,g,sg=GGI(h,w,search=['S','G'],replacement_of_found='.',mp_def={'#':1,'.':0},boundary=1) # sample usage
    mp=[boundary]*(w+2);found={}
    for i in range(h):
        s=input()
        for char in search:
            if char in s:
                found[char]=((i+1)*(w+2)+s.index(char)+1)
                mp_def[char]=mp_def[replacement_of_found]
        mp+=[boundary]+[mp_def[j] for j in s]+[boundary]
    mp+=[boundary]*(w+2)
    return h+2,w+2,mp,found
def TI(n):return GI(n,n-1)
def accum(ls):
    rt=[0]
    for i in ls:rt+=[rt[-1]+i]
    return rt
def bit_combination(n,base=2):
    rt=[]
    for tb in range(base**n):s=[tb//(base**bt)%base for bt in range(n)];rt+=[s]
    return rt
def gcd(x,y):
    if y==0:return x
    if x%y==0:return y
    while x%y!=0:x,y=y,x%y
    return y
def YN(x):print(['NO','YES'][x])
def Yn(x):print(['No','Yes'][x])
def show(*inp,end='\n'):
    if show_flg:print(*inp,end=end)

mo=10**9+7
inf=float('inf')
FourNb=[(-1,0),(1,0),(0,1),(0,-1)];EightNb=[(-1,0),(1,0),(0,1),(0,-1),(1,1),(-1,-1),(1,-1),(-1,1)];compas=dict(zip('WENS',FourNb));cursol=dict(zip('LRUD',FourNb))
l_alp=string.ascii_lowercase
#sys.setrecursionlimit(10**7)
read=sys.stdin.buffer.read
readline=sys.stdin.buffer.readline
input=lambda: sys.stdin.readline().rstrip()

class Tree:
    def __init__(self,inp_size=None,ls=None,init=True,index=1):
        self.LCA_init_stat=False
        self.ETtable=[]
        if init:
            if ls==None:
                self.stdin(inp_size,index=index)
            else:
                self.node_size=len(ls)+1
                self.edges,_=GI(self.node_size,self.node_size-1,ls,index=index)
        return
 
    def stdin(self,inp_size=None,index=1):
        if inp_size==None:
            self.node_size=int(input())
        else:
            self.node_size=inp_size
        self.edges,_=GI(self.node_size,self.node_size-1,index=index)
        return
    
    def listin(self,ls,index=0):
        self.node_size=len(ls)+1
        self.edges,_=GI(self.node_size,self.node_size-1,ls,index=index)
        return
 
    def dfs(self,x,func=lambda pr,prv,nx,dist:prv+dist,root_v=0):
        q=deque([x])
        v=[None]*self.node_size
        v[x]=root_v
        while q:
            c=q.pop()
            for nb,d in self.edges[c]:
                if v[nb]==None:
                    q.append(nb)
                    v[nb]=func(c,v[c],nb,d)
        return v
 
    def bfs(self,x,func=lambda pr,prv,nx,dist:prv+dist,root_v=0):
        q=deque([x])
        v=[None]*self.node_size
        v[x]=root_v
        while q:
            c=q.popleft()
            for nb,d in self.edges[c]:
                if v[nb]==None:
                    q.append(nb)
                    v[nb]=func(c,v[c],nb,d)
        return v
 
    def parent(self,x):
        return self.dfs(0,func=lambda pr,prv,nx,dist:pr,root_v=-1)
 
    def topological_sort(self,x):  # return topological sort of the tree
        tps=[]
        q=deque([x])
        v=[None]*self.node_size
        v[x]=0
        while q:
            c=q.popleft()
            tps.append(c)
            for nb,d in self.edges[c]:
                if v[nb]==None:
                    q.append(nb)
                    v[nb]=0
        return tps
 
    def EulerTour(self,x):
        q=deque()
        q.append(x)
        self.depth=[None]*self.node_size
        self.depth[x]=0
        self.ETtable=[]
        self.ETdepth=[]
        self.ETin=[-1]*self.node_size
        self.ETout=[-1]*self.node_size
        cnt=0
        while q:
            c=q.pop()
            if c<0:
                ce=~c
            else:
                ce=c
                for nb,d in self.edges[ce]:
                    if self.depth[nb]==None:
                        q.append(~ce)
                        q.append(nb)
                        self.depth[nb]=self.depth[ce]+1
            self.ETtable.append(ce)
            self.ETdepth.append(self.depth[ce])
            if self.ETin[ce]==-1:
                self.ETin[ce]=cnt
            else:
                self.ETout[ce]=cnt
            cnt+=1
        return
    
    def LCA_init(self,root):
        self.EulerTour(root)
        self.st=SparseTable(self.ETdepth,init_func=min,init_idl=inf)
        #self.st=SegTree(self.node_size*2-1,self.ETdepth,function=min,ide=inf)
        self.LCA_init_stat=True
        return
    
    def LCA(self,root,x,y):
        if self.LCA_init_stat==False:
            self.LCA_init(root)
        xin,xout=self.ETin[x],self.ETout[x]
        yin,yout=self.ETin[y],self.ETout[y]
        a=min(xin,yin)
        b=max(xout,yout,xin,yin)
        id_of_min_dep_in_et=self.st.query_id(a,b+1)
        return self.ETtable[id_of_min_dep_in_et]
 
    def __str__(self):
        return  str(self.edges)
 
    def show(self):
        if all([all([d==1 for nd,d in e]) for e in self.edges]):
            print( [[nd for nd,d in e] for e in self.edges] )
        else:
            print(self)

class SparseTable: # O(N log N) for init, O(1) for query(l,r)
    def __init__(self,ls,init_func=min,init_idl=float('inf')):
        self.func=init_func
        self.idl=init_idl
        self.size=len(ls)
        self.N0=self.size.bit_length()
        self.table=[ls[:]]
        self.index=[list(range(self.size))]
        self.lg=[0]*(self.size+1)
        
        for i in range(2,self.size+1):
            self.lg[i]=self.lg[i>>1]+1
 
        for i in range(self.N0):
            tmp=[self.func(self.table[i][j],self.table[i][min(j+(1<<i),self.size-1)]) for j in range(self.size)]
            tmp_id=[self.index[i][j] if self.table[i][j]==self.func(self.table[i][j],self.table[i][min(j+(1<<i),self.size-1)]) else self.index[i][min(j+(1<<i),self.size-1)] for j in range(self.size)]
            self.table+=[tmp]
            self.index+=[tmp_id]
    
    # return func of [l,r)
    def query(self,l,r):
        if r>self.size:r=self.size
        #N=(r-l).bit_length()-1
        N=self.lg[r-l]
        return self.func(self.table[N][l],self.table[N][max(0,r-(1<<N))])
    
    # return index of which val[i] = func of v among [l,r)
    def query_id(self,l,r):
        if r>self.size:r=self.size
        #N=(r-l).bit_length()-1
        N=self.lg[r-l]
        a,b=self.index[N][l],self.index[N][max(0,r-(1<<N))]
        if self.table[0][a]==self.func(self.table[N][l],self.table[N][max(0,r-(1<<N))]):
            b=a
        return b
    
    # return boundary index of r such that func({table[i]} =< x , i in [pos,r]
    def right_bound(self,pos,x):
        k=(self.size-pos).bit_length()
        for j in range(k)[::-1]:
            nx=pos+(1<<j)
            if nx<n and self.query(pos,nx+1)<=x:
                pos+=(1<<j)
        return pos
    
    # return boundary index of l such that func({table[i]} =< x , i in [l,pos]
    def left_bound(self,pos,x):
        k=pos.bit_length()
        for j in range(k)[::-1]:
            nx=pos-(1<<j)
            if 0<=nx and self.query(nx,pos+1)<=x:
                pos-=(1<<j)
        return pos
 
    def __str__(self):
        return str(self.table[0])
 
    def print(self):
        for i in self.table:
            print(*i)


show_flg=False
show_flg=True
ans=0


class Graph:
    def __init__(self,inp_node_size=None,inp_edge_size=None,ls=None,init=True,Directed=False,index=1):
        if init:
            self.IsGrid=False
            if ls==None:
                self.stdin(inp_node_size,inp_edge_size,Directed=Directed,index=index)
            else:
                self.listin(ls,Directed=Directed,index=index)
        else:
            self.edges=[]
        return

    def stdin(self,inp_node_size=None,inp_edge_size=None,Directed=False,index=1):
        if (inp_node_size,inp_edge_size)==(None,None):
            self.node_size,self.edge_size=LI()
            self.edges,_=GI(self.node_size,self.edge_size,Directed=Directed,index=index)
        else:
            self.node_size,self.edge_size=inp_node_size,inp_edge_size
            self.edges,_=GI(self.node_size,self.edge_size,Directed=Directed,index=index)
        return

    def listin(self,inp_node_size,ls,Directed=False,index=0):
        self.node_size,self.edge_size=inp_node_size,len(ls)
        self.edges,_=GI(self.node_size,self.edge_size,ls,Directed=Directed,index=index)
        return

    def edgein(self,inp_edge,index=0):
        self.node_size,self.edge_size=len(inp_edge),0
        self.edges=[0]*self.node_size
        for i in range(self.node_size):
            self.edges[i]=inp_edge[i][:]
        return

    def gridin(self,h,w,search=None,replacement_of_found='.',mp_def={'#':1,'.':0}):
        #h,w,g,sg=GGI(h,w,search=['S','G'],replacement_of_found='.',mp_def={'#':1,'.':0},boundary=1) # sample usage
        self.mp,found=[],{}
        if search==None:
            search=[]
        self.node_size,self.edge_size,self.grid_h,self.grid_w=h*w,0,h,w
        self.edges=[[] for i in range(self.node_size)]
        self.IsGrid=True
        for i in range(h):
            s=input()
            for char in search:
                if char in s:
                    found[char]=(i*w+s.index(char))
                    mp_def[char]=mp_def[replacement_of_found]
            self.mp+=[mp_def[j] for j in s]
        for dd in [1,w]:
            for node in range(self.node_size):
                nx=node+dd
                if nx<0 or self.node_size<=nx or (dd==1 and nx%w==0):
                    continue
                if self.mp[nx]==0:
                    self.edges[node].append((nx,1))
                if self.mp[node]==0:
                    self.edges[nx].append((node,1))
        return found

    def dfs(self,x,func=lambda pr,prv,nx,dist:prv+dist,root_v=0):
        q=deque([x])
        v=[None]*self.node_size
        v[x]=root_v
        while q:
            c=q.pop()
            for nb,d in self.edges[c]:
                if v[nb]==None:
                    q.append(nb)
                    v[nb]=func(c,v[c],nb,d)
        return v

    def bfs(self,x,func=lambda pr,prv,nx,dist:prv+dist,root_v=0):
        q=deque([x])
        q=deque([i for i,j in enumerate(self.mp) if j==1])
        v=[None]*self.node_size
        #v[x]=root_v
        for y in q:v[y]=root_v
        while q:
            c=q.popleft()
            for nb,d in self.edges[c]:
                if v[nb]==None:
                    q.append(nb)
                    v[nb]=func(c,v[c],nb,d)
        return v

    def parent(self,x):
        return self.bfs(0,func=lambda pr,prv,nx,dist:pr,root_v=-1)

    def topological_sort(self,x):  # return topological sort of the tree
        tps=[]
        q=deque([x])
        v=[None]*self.node_size
        v[x]=0
        while q:
            c=q.popleft()
            tps.append(c)
            for nb,d in self.edges[c]:
                if v[nb]==None:
                    q.append(nb)
                    v[nb]=0
        return tps

    def dijkstra(self,st):
        # edge=[[(v_to,dist_to_v),...],[],...]
        # initialize: def: d=dist(st,i), prev=[previous vertex in minimum path], q[]
        d=[(0 if st==i else inf) for i in range(self.node_size)]
        prev=[0]*self.node_size
        q=[(j,i) for i,j in enumerate(d)]
        heapify(q)
        v=[False]*self.node_size
        # calc
        while q:
            dist,cur=heappop(q)
            if v[cur]:
                continue
            v[cur]=True
            for dst,dist in self.edges[cur]:
                alt=d[cur]+dist
                if alt<d[dst]:
                    d[dst]=alt
                    prev[dst]=cur
                    heappush(q,(alt,dst))
        return d,prev
    
    def __str__(self):
        if self.IsGrid:
            for i in range(self.grid_h):
                print(*self.mp[i*self.grid_w:-~i*self.grid_w])
            return str(str(self.show()))
        else:
            return  str(str(self.show()))

    def show(self):  # hide distance when all distance == 1
        if self.IsGrid:
            rt=[[nd for nd,d in e] for e in self.edges]
            return 'Grid',rt
        else:
            if all([all([d==1 for nd,d in e]) for e in self.edges]):
                rt=[[nd for nd,d in e] for e in self.edges]
                return 'Simplified',rt
            else:
                return self


gr=Graph(init=False)
h,w=LI()

f=gr.gridin(h,w)
v=gr.bfs(0)

print(max(v))




Fixed submission:
from collections import deque
h,w=map(int,input().split())
a=[[] for _ in range(h)]
for i in range(h):
    s=input()
    for j in s:
        if j=='.':
            a[i].append(-1)
        else:
            a[i].append(0)

def bfs():
    d=[[0,1],[1,0],[-1,0],[0,-1]]
    q=deque()
    c=0
    for i in range(h):
        for j in range(w):
            if a[i][j]==0:
                q.append((i,j))
    while(len(q)>0):
        cur=q.popleft()
        i,j=cur
        for dd in d:
            di,dj=dd[0],dd[1]
            if (i+di)*(h-1-i-di)>=0 and (j+dj)*(w-1-j-dj)>=0 and a[i+di][j+dj]<0:
                c=a[i][j]+1
                a[i+di][j+dj]=c
                q.append((i+di,j+dj))
    return(c)

print(bfs())

Buggy python submission (verdict: Time Limit Exceeded):
import sys,bisect,string,math,time,functools,random,fractions
from heapq import heappush,heappop,heapify
from collections import deque,defaultdict,Counter
from itertools import permutations,combinations,groupby
rep=range
def Golf():n,*t=map(int,open(0).read().split())
def I():return int(input())
def S_():return input()
def IS():return input().split()
def LS():return [i for i in input().split()]
def MI():return map(int,input().split())
def LI():return [int(i) for i in input().split()]
def LI_():return [int(i)-1 for i in input().split()]
def NI(n):return [int(input()) for i in range(n)]
def NI_(n):return [int(input())-1 for i in range(n)]
def StoLI():return [ord(i)-97 for i in input()]
def ItoS(n):return chr(n+97)
def LtoS(ls):return ''.join([chr(i+97) for i in ls])
def RA():return map(int,open(0).read().split())
def GI(V,E,ls=None,Directed=False,index=1):
    org_inp=[];g=[[] for i in range(V)]
    FromStdin=True if ls==None else False
    for i in range(E):
        if FromStdin:
            inp=LI()
            org_inp.append(inp)
        else:
            inp=ls[i]
        if len(inp)==2:
            a,b=inp;c=1
        else:
            a,b,c=inp
        if index==1:a-=1;b-=1
        aa=(a,c);bb=(b,c);g[a].append(bb)
        if not Directed:g[b].append(aa)
    return g,org_inp
def GGI(h,w,search=None,replacement_of_found='.',mp_def={'#':1,'.':0},boundary=1):
    #h,w,g,sg=GGI(h,w,search=['S','G'],replacement_of_found='.',mp_def={'#':1,'.':0},boundary=1) # sample usage
    mp=[boundary]*(w+2);found={}
    for i in range(h):
        s=input()
        for char in search:
            if char in s:
                found[char]=((i+1)*(w+2)+s.index(char)+1)
                mp_def[char]=mp_def[replacement_of_found]
        mp+=[boundary]+[mp_def[j] for j in s]+[boundary]
    mp+=[boundary]*(w+2)
    return h+2,w+2,mp,found
def TI(n):return GI(n,n-1)
def accum(ls):
    rt=[0]
    for i in ls:rt+=[rt[-1]+i]
    return rt
def bit_combination(n,base=2):
    rt=[]
    for tb in range(base**n):s=[tb//(base**bt)%base for bt in range(n)];rt+=[s]
    return rt
def gcd(x,y):
    if y==0:return x
    if x%y==0:return y
    while x%y!=0:x,y=y,x%y
    return y
def YN(x):print(['NO','YES'][x])
def Yn(x):print(['No','Yes'][x])
def show(*inp,end='\n'):
    if show_flg:print(*inp,end=end)

mo=10**9+7
inf=float('inf')
FourNb=[(-1,0),(1,0),(0,1),(0,-1)];EightNb=[(-1,0),(1,0),(0,1),(0,-1),(1,1),(-1,-1),(1,-1),(-1,1)];compas=dict(zip('WENS',FourNb));cursol=dict(zip('LRUD',FourNb))
l_alp=string.ascii_lowercase
#sys.setrecursionlimit(10**7)
read=sys.stdin.buffer.read
readline=sys.stdin.buffer.readline
input=lambda: sys.stdin.readline().rstrip()

class Tree:
    def __init__(self,inp_size=None,ls=None,init=True,index=1):
        self.LCA_init_stat=False
        self.ETtable=[]
        if init:
            if ls==None:
                self.stdin(inp_size,index=index)
            else:
                self.node_size=len(ls)+1
                self.edges,_=GI(self.node_size,self.node_size-1,ls,index=index)
        return
 
    def stdin(self,inp_size=None,index=1):
        if inp_size==None:
            self.node_size=int(input())
        else:
            self.node_size=inp_size
        self.edges,_=GI(self.node_size,self.node_size-1,index=index)
        return
    
    def listin(self,ls,index=0):
        self.node_size=len(ls)+1
        self.edges,_=GI(self.node_size,self.node_size-1,ls,index=index)
        return
 
    def dfs(self,x,func=lambda pr,prv,nx,dist:prv+dist,root_v=0):
        q=deque([x])
        v=[None]*self.node_size
        v[x]=root_v
        while q:
            c=q.pop()
            for nb,d in self.edges[c]:
                if v[nb]==None:
                    q.append(nb)
                    v[nb]=func(c,v[c],nb,d)
        return v
 
    def bfs(self,x,func=lambda pr,prv,nx,dist:prv+dist,root_v=0):
        q=deque([x])
        v=[None]*self.node_size
        v[x]=root_v
        while q:
            c=q.popleft()
            for nb,d in self.edges[c]:
                if v[nb]==None:
                    q.append(nb)
                    v[nb]=func(c,v[c],nb,d)
        return v
 
    def parent(self,x):
        return self.dfs(0,func=lambda pr,prv,nx,dist:pr,root_v=-1)
 
    def topological_sort(self,x):  # return topological sort of the tree
        tps=[]
        q=deque([x])
        v=[None]*self.node_size
        v[x]=0
        while q:
            c=q.popleft()
            tps.append(c)
            for nb,d in self.edges[c]:
                if v[nb]==None:
                    q.append(nb)
                    v[nb]=0
        return tps
 
    def EulerTour(self,x):
        q=deque()
        q.append(x)
        self.depth=[None]*self.node_size
        self.depth[x]=0
        self.ETtable=[]
        self.ETdepth=[]
        self.ETin=[-1]*self.node_size
        self.ETout=[-1]*self.node_size
        cnt=0
        while q:
            c=q.pop()
            if c<0:
                ce=~c
            else:
                ce=c
                for nb,d in self.edges[ce]:
                    if self.depth[nb]==None:
                        q.append(~ce)
                        q.append(nb)
                        self.depth[nb]=self.depth[ce]+1
            self.ETtable.append(ce)
            self.ETdepth.append(self.depth[ce])
            if self.ETin[ce]==-1:
                self.ETin[ce]=cnt
            else:
                self.ETout[ce]=cnt
            cnt+=1
        return
    
    def LCA_init(self,root):
        self.EulerTour(root)
        self.st=SparseTable(self.ETdepth,init_func=min,init_idl=inf)
        #self.st=SegTree(self.node_size*2-1,self.ETdepth,function=min,ide=inf)
        self.LCA_init_stat=True
        return
    
    def LCA(self,root,x,y):
        if self.LCA_init_stat==False:
            self.LCA_init(root)
        xin,xout=self.ETin[x],self.ETout[x]
        yin,yout=self.ETin[y],self.ETout[y]
        a=min(xin,yin)
        b=max(xout,yout,xin,yin)
        id_of_min_dep_in_et=self.st.query_id(a,b+1)
        return self.ETtable[id_of_min_dep_in_et]
 
    def __str__(self):
        return  str(self.edges)
 
    def show(self):
        if all([all([d==1 for nd,d in e]) for e in self.edges]):
            print( [[nd for nd,d in e] for e in self.edges] )
        else:
            print(self)

class SparseTable: # O(N log N) for init, O(1) for query(l,r)
    def __init__(self,ls,init_func=min,init_idl=float('inf')):
        self.func=init_func
        self.idl=init_idl
        self.size=len(ls)
        self.N0=self.size.bit_length()
        self.table=[ls[:]]
        self.index=[list(range(self.size))]
        self.lg=[0]*(self.size+1)
        
        for i in range(2,self.size+1):
            self.lg[i]=self.lg[i>>1]+1
 
        for i in range(self.N0):
            tmp=[self.func(self.table[i][j],self.table[i][min(j+(1<<i),self.size-1)]) for j in range(self.size)]
            tmp_id=[self.index[i][j] if self.table[i][j]==self.func(self.table[i][j],self.table[i][min(j+(1<<i),self.size-1)]) else self.index[i][min(j+(1<<i),self.size-1)] for j in range(self.size)]
            self.table+=[tmp]
            self.index+=[tmp_id]
    
    # return func of [l,r)
    def query(self,l,r):
        if r>self.size:r=self.size
        #N=(r-l).bit_length()-1
        N=self.lg[r-l]
        return self.func(self.table[N][l],self.table[N][max(0,r-(1<<N))])
    
    # return index of which val[i] = func of v among [l,r)
    def query_id(self,l,r):
        if r>self.size:r=self.size
        #N=(r-l).bit_length()-1
        N=self.lg[r-l]
        a,b=self.index[N][l],self.index[N][max(0,r-(1<<N))]
        if self.table[0][a]==self.func(self.table[N][l],self.table[N][max(0,r-(1<<N))]):
            b=a
        return b
    
    # return boundary index of r such that func({table[i]} =< x , i in [pos,r]
    def right_bound(self,pos,x):
        k=(self.size-pos).bit_length()
        for j in range(k)[::-1]:
            nx=pos+(1<<j)
            if nx<n and self.query(pos,nx+1)<=x:
                pos+=(1<<j)
        return pos
    
    # return boundary index of l such that func({table[i]} =< x , i in [l,pos]
    def left_bound(self,pos,x):
        k=pos.bit_length()
        for j in range(k)[::-1]:
            nx=pos-(1<<j)
            if 0<=nx and self.query(nx,pos+1)<=x:
                pos-=(1<<j)
        return pos
 
    def __str__(self):
        return str(self.table[0])
 
    def print(self):
        for i in self.table:
            print(*i)


show_flg=False
show_flg=True
ans=0


class Graph:
    def __init__(self,inp_node_size=None,inp_edge_size=None,ls=None,init=True,Directed=False,index=1):
        if init:
            self.IsGrid=False
            if ls==None:
                self.stdin(inp_node_size,inp_edge_size,Directed=Directed,index=index)
            else:
                self.listin(ls,Directed=Directed,index=index)
        else:
            self.edges=[]
        return

    def stdin(self,inp_node_size=None,inp_edge_size=None,Directed=False,index=1):
        if (inp_node_size,inp_edge_size)==(None,None):
            self.node_size,self.edge_size=LI()
            self.edges,_=GI(self.node_size,self.edge_size,Directed=Directed,index=index)
        else:
            self.node_size,self.edge_size=inp_node_size,inp_edge_size
            self.edges,_=GI(self.node_size,self.edge_size,Directed=Directed,index=index)
        return

    def listin(self,inp_node_size,ls,Directed=False,index=0):
        self.node_size,self.edge_size=inp_node_size,len(ls)
        self.edges,_=GI(self.node_size,self.edge_size,ls,Directed=Directed,index=index)
        return

    def edgein(self,inp_edge,index=0):
        self.node_size,self.edge_size=len(inp_edge),0
        self.edges=[0]*self.node_size
        for i in range(self.node_size):
            self.edges[i]=inp_edge[i][:]
        return

    def gridin(self,h,w,search=None,replacement_of_found='.',mp_def={'#':1,'.':0}):
        #h,w,g,sg=GGI(h,w,search=['S','G'],replacement_of_found='.',mp_def={'#':1,'.':0},boundary=1) # sample usage
        self.mp,found=[],{}
        if search==None:
            search=[]
        self.node_size,self.edge_size,self.grid_h,self.grid_w=h*w,0,h,w
        self.edges=[[] for i in range(self.node_size)]
        self.IsGrid=True
        for i in range(h):
            s=input()
            for char in search:
                if char in s:
                    found[char]=(i*w+s.index(char))
                    mp_def[char]=mp_def[replacement_of_found]
            self.mp+=[mp_def[j] for j in s]
        for dd in [1,w]:
            for node in range(self.node_size):
                H,W=node//w,node%w
                nx=node+dd
                if nx<0 or self.node_size<=nx or (dd==1 and nx%w==0):
                    continue
                if self.mp[nx]==0:
                    self.edges[node].append((nx,1))
                if self.mp[node]==0:
                    self.edges[nx].append((node,1))
        return found

    def dfs(self,x,func=lambda pr,prv,nx,dist:prv+dist,root_v=0):
        q=deque([x])
        v=[None]*self.node_size
        v[x]=root_v
        while q:
            c=q.pop()
            for nb,d in self.edges[c]:
                if v[nb]==None:
                    q.append(nb)
                    v[nb]=func(c,v[c],nb,d)
        return v

    def bfs(self,x,func=lambda pr,prv,nx,dist:prv+dist,root_v=0):
        q=deque([x])
        q=deque([i for i,j in enumerate(self.mp) if j==1])
        v=[None]*self.node_size
        #v[x]=root_v
        for y in q:v[y]=root_v
        while q:
            c=q.popleft()
            for nb,d in self.edges[c]:
                if v[nb]==None:
                    q.append(nb)
                    v[nb]=func(c,v[c],nb,d)
        return v

    def parent(self,x):
        return self.bfs(0,func=lambda pr,prv,nx,dist:pr,root_v=-1)

    def topological_sort(self,x):  # return topological sort of the tree
        tps=[]
        q=deque([x])
        v=[None]*self.node_size
        v[x]=0
        while q:
            c=q.popleft()
            tps.append(c)
            for nb,d in self.edges[c]:
                if v[nb]==None:
                    q.append(nb)
                    v[nb]=0
        return tps

    def dijkstra(self,st):
        # edge=[[(v_to,dist_to_v),...],[],...]
        # initialize: def: d=dist(st,i), prev=[previous vertex in minimum path], q[]
        d=[(0 if st==i else inf) for i in range(self.node_size)]
        prev=[0]*self.node_size
        q=[(j,i) for i,j in enumerate(d)]
        heapify(q)
        v=[False]*self.node_size
        # calc
        while q:
            dist,cur=heappop(q)
            if v[cur]:
                continue
            v[cur]=True
            for dst,dist in self.edges[cur]:
                alt=d[cur]+dist
                if alt<d[dst]:
                    d[dst]=alt
                    prev[dst]=cur
                    heappush(q,(alt,dst))
        return d,prev
    
    def __str__(self):
        if self.IsGrid:
            for i in range(self.grid_h):
                print(*self.mp[i*self.grid_w:-~i*self.grid_w])
            return str(str(self.show()))
        else:
            return  str(str(self.show()))

    def show(self):  # hide distance when all distance == 1
        if self.IsGrid:
            rt=[[nd for nd,d in e] for e in self.edges]
            return 'Grid',rt
        else:
            if all([all([d==1 for nd,d in e]) for e in self.edges]):
                rt=[[nd for nd,d in e] for e in self.edges]
                return 'Simplified',rt
            else:
                return self


gr=Graph(init=False)
h,w=LI()

f=gr.gridin(h,w)
v=gr.bfs(0)

print(max(v))




Fixed submission:
from collections import deque
h,w=map(int,input().split())
a=[[] for _ in range(h)]
for i in range(h):
    s=input()
    for j in s:
        if j=='.':
            a[i].append(-1)
        else:
            a[i].append(0)

def bfs():
    d=[[0,1],[1,0],[-1,0],[0,-1]]
    q=deque()
    c=0
    for i in range(h):
        for j in range(w):
            if a[i][j]==0:
                q.append((i,j))
    while(len(q)>0):
        cur=q.popleft()
        i,j=cur
        for dd in d:
            di,dj=dd[0],dd[1]
            if (i+di)*(h-1-i-di)>=0 and (j+dj)*(w-1-j-dj)>=0 and a[i+di][j+dj]<0:
                c=a[i][j]+1
                a[i+di][j+dj]=c
                q.append((i+di,j+dj))
    return(c)

print(bfs())

Buggy python submission (verdict: Runtime Error):
import sys,bisect,string,math,time,functools,random,fractions
from heapq import heappush,heappop,heapify
from collections import deque,defaultdict,Counter
from itertools import permutations,combinations,groupby
rep=range
def Golf():n,*t=map(int,open(0).read().split())
def I():return int(input())
def S_():return input()
def IS():return input().split()
def LS():return [i for i in input().split()]
def MI():return map(int,input().split())
def LI():return [int(i) for i in input().split()]
def LI_():return [int(i)-1 for i in input().split()]
def NI(n):return [int(input()) for i in range(n)]
def NI_(n):return [int(input())-1 for i in range(n)]
def StoLI():return [ord(i)-97 for i in input()]
def ItoS(n):return chr(n+97)
def LtoS(ls):return ''.join([chr(i+97) for i in ls])
def RA():return map(int,open(0).read().split())
def GI(V,E,ls=None,Directed=False,index=1):
    org_inp=[];g=[[] for i in range(V)]
    FromStdin=True if ls==None else False
    for i in range(E):
        if FromStdin:
            inp=LI()
            org_inp.append(inp)
        else:
            inp=ls[i]
        if len(inp)==2:
            a,b=inp;c=1
        else:
            a,b,c=inp
        if index==1:a-=1;b-=1
        aa=(a,c);bb=(b,c);g[a].append(bb)
        if not Directed:g[b].append(aa)
    return g,org_inp
def GGI(h,w,search=None,replacement_of_found='.',mp_def={'#':1,'.':0},boundary=1):
    #h,w,g,sg=GGI(h,w,search=['S','G'],replacement_of_found='.',mp_def={'#':1,'.':0},boundary=1) # sample usage
    mp=[boundary]*(w+2);found={}
    for i in range(h):
        s=input()
        for char in search:
            if char in s:
                found[char]=((i+1)*(w+2)+s.index(char)+1)
                mp_def[char]=mp_def[replacement_of_found]
        mp+=[boundary]+[mp_def[j] for j in s]+[boundary]
    mp+=[boundary]*(w+2)
    return h+2,w+2,mp,found
def TI(n):return GI(n,n-1)
def accum(ls):
    rt=[0]
    for i in ls:rt+=[rt[-1]+i]
    return rt
def bit_combination(n,base=2):
    rt=[]
    for tb in range(base**n):s=[tb//(base**bt)%base for bt in range(n)];rt+=[s]
    return rt
def gcd(x,y):
    if y==0:return x
    if x%y==0:return y
    while x%y!=0:x,y=y,x%y
    return y
def YN(x):print(['NO','YES'][x])
def Yn(x):print(['No','Yes'][x])
def show(*inp,end='\n'):
    if show_flg:print(*inp,end=end)

mo=10**9+7
inf=float('inf')
FourNb=[(-1,0),(1,0),(0,1),(0,-1)];EightNb=[(-1,0),(1,0),(0,1),(0,-1),(1,1),(-1,-1),(1,-1),(-1,1)];compas=dict(zip('WENS',FourNb));cursol=dict(zip('LRUD',FourNb))
l_alp=string.ascii_lowercase
#sys.setrecursionlimit(10**7)
read=sys.stdin.buffer.read
readline=sys.stdin.buffer.readline
input=lambda: sys.stdin.readline().rstrip()

class Tree:
    def __init__(self,inp_size=None,ls=None,init=True,index=1):
        self.LCA_init_stat=False
        self.ETtable=[]
        if init:
            if ls==None:
                self.stdin(inp_size,index=index)
            else:
                self.node_size=len(ls)+1
                self.edges,_=GI(self.node_size,self.node_size-1,ls,index=index)
        return
 
    def stdin(self,inp_size=None,index=1):
        if inp_size==None:
            self.node_size=int(input())
        else:
            self.node_size=inp_size
        self.edges,_=GI(self.node_size,self.node_size-1,index=index)
        return
    
    def listin(self,ls,index=0):
        self.node_size=len(ls)+1
        self.edges,_=GI(self.node_size,self.node_size-1,ls,index=index)
        return
 
    def dfs(self,x,func=lambda pr,prv,nx,dist:prv+dist,root_v=0):
        q=deque([x])
        v=[None]*self.node_size
        v[x]=root_v
        while q:
            c=q.pop()
            for nb,d in self.edges[c]:
                if v[nb]==None:
                    q.append(nb)
                    v[nb]=func(c,v[c],nb,d)
        return v
 
    def bfs(self,x,func=lambda pr,prv,nx,dist:prv+dist,root_v=0):
        q=deque([x])
        v=[None]*self.node_size
        v[x]=root_v
        while q:
            c=q.popleft()
            for nb,d in self.edges[c]:
                if v[nb]==None:
                    q.append(nb)
                    v[nb]=func(c,v[c],nb,d)
        return v
 
    def parent(self,x):
        return self.dfs(0,func=lambda pr,prv,nx,dist:pr,root_v=-1)
 
    def topological_sort(self,x):  # return topological sort of the tree
        tps=[]
        q=deque([x])
        v=[None]*self.node_size
        v[x]=0
        while q:
            c=q.popleft()
            tps.append(c)
            for nb,d in self.edges[c]:
                if v[nb]==None:
                    q.append(nb)
                    v[nb]=0
        return tps
 
    def EulerTour(self,x):
        q=deque()
        q.append(x)
        self.depth=[None]*self.node_size
        self.depth[x]=0
        self.ETtable=[]
        self.ETdepth=[]
        self.ETin=[-1]*self.node_size
        self.ETout=[-1]*self.node_size
        cnt=0
        while q:
            c=q.pop()
            if c<0:
                ce=~c
            else:
                ce=c
                for nb,d in self.edges[ce]:
                    if self.depth[nb]==None:
                        q.append(~ce)
                        q.append(nb)
                        self.depth[nb]=self.depth[ce]+1
            self.ETtable.append(ce)
            self.ETdepth.append(self.depth[ce])
            if self.ETin[ce]==-1:
                self.ETin[ce]=cnt
            else:
                self.ETout[ce]=cnt
            cnt+=1
        return
    
    def LCA_init(self,root):
        self.EulerTour(root)
        self.st=SparseTable(self.ETdepth,init_func=min,init_idl=inf)
        #self.st=SegTree(self.node_size*2-1,self.ETdepth,function=min,ide=inf)
        self.LCA_init_stat=True
        return
    
    def LCA(self,root,x,y):
        if self.LCA_init_stat==False:
            self.LCA_init(root)
        xin,xout=self.ETin[x],self.ETout[x]
        yin,yout=self.ETin[y],self.ETout[y]
        a=min(xin,yin)
        b=max(xout,yout,xin,yin)
        id_of_min_dep_in_et=self.st.query_id(a,b+1)
        return self.ETtable[id_of_min_dep_in_et]
 
    def __str__(self):
        return  str(self.edges)
 
    def show(self):
        if all([all([d==1 for nd,d in e]) for e in self.edges]):
            print( [[nd for nd,d in e] for e in self.edges] )
        else:
            print(self)

class SparseTable: # O(N log N) for init, O(1) for query(l,r)
    def __init__(self,ls,init_func=min,init_idl=float('inf')):
        self.func=init_func
        self.idl=init_idl
        self.size=len(ls)
        self.N0=self.size.bit_length()
        self.table=[ls[:]]
        self.index=[list(range(self.size))]
        self.lg=[0]*(self.size+1)
        
        for i in range(2,self.size+1):
            self.lg[i]=self.lg[i>>1]+1
 
        for i in range(self.N0):
            tmp=[self.func(self.table[i][j],self.table[i][min(j+(1<<i),self.size-1)]) for j in range(self.size)]
            tmp_id=[self.index[i][j] if self.table[i][j]==self.func(self.table[i][j],self.table[i][min(j+(1<<i),self.size-1)]) else self.index[i][min(j+(1<<i),self.size-1)] for j in range(self.size)]
            self.table+=[tmp]
            self.index+=[tmp_id]
    
    # return func of [l,r)
    def query(self,l,r):
        if r>self.size:r=self.size
        #N=(r-l).bit_length()-1
        N=self.lg[r-l]
        return self.func(self.table[N][l],self.table[N][max(0,r-(1<<N))])
    
    # return index of which val[i] = func of v among [l,r)
    def query_id(self,l,r):
        if r>self.size:r=self.size
        #N=(r-l).bit_length()-1
        N=self.lg[r-l]
        a,b=self.index[N][l],self.index[N][max(0,r-(1<<N))]
        if self.table[0][a]==self.func(self.table[N][l],self.table[N][max(0,r-(1<<N))]):
            b=a
        return b
    
    # return boundary index of r such that func({table[i]} =< x , i in [pos,r]
    def right_bound(self,pos,x):
        k=(self.size-pos).bit_length()
        for j in range(k)[::-1]:
            nx=pos+(1<<j)
            if nx<n and self.query(pos,nx+1)<=x:
                pos+=(1<<j)
        return pos
    
    # return boundary index of l such that func({table[i]} =< x , i in [l,pos]
    def left_bound(self,pos,x):
        k=pos.bit_length()
        for j in range(k)[::-1]:
            nx=pos-(1<<j)
            if 0<=nx and self.query(nx,pos+1)<=x:
                pos-=(1<<j)
        return pos
 
    def __str__(self):
        return str(self.table[0])
 
    def print(self):
        for i in self.table:
            print(*i)


show_flg=False
show_flg=True
ans=0


class Graph:
    def __init__(self,inp_node_size=None,inp_edge_size=None,ls=None,init=True,Directed=False,index=1):
        if init:
            self.IsGrid=False
            if ls==None:
                self.stdin(inp_node_size,inp_edge_size,Directed=Directed,index=index)
            else:
                self.listin(ls,Directed=Directed,index=index)
        else:
            self.edges=[]
        return

    def stdin(self,inp_node_size=None,inp_edge_size=None,Directed=False,index=1):
        if (inp_node_size,inp_edge_size)==(None,None):
            self.node_size,self.edge_size=LI()
            self.edges,_=GI(self.node_size,self.edge_size,Directed=Directed,index=index)
        else:
            self.node_size,self.edge_size=inp_node_size,inp_edge_size
            self.edges,_=GI(self.node_size,self.edge_size,Directed=Directed,index=index)
        return

    def listin(self,inp_node_size,ls,Directed=False,index=0):
        self.node_size,self.edge_size=inp_node_size,len(ls)
        self.edges,_=GI(self.node_size,self.edge_size,ls,Directed=Directed,index=index)
        return

    def edgein(self,inp_edge,index=0):
        self.node_size,self.edge_size=len(inp_edge),0
        self.edges=[0]*self.node_size
        for i in range(self.node_size):
            self.edges[i]=inp_edge[i][:]
        return

    def gridin(self,h,w,search=None,replacement_of_found='.',mp_def={'#':1,'.':0}):
        #h,w,g,sg=GGI(h,w,search=['S','G'],replacement_of_found='.',mp_def={'#':1,'.':0},boundary=1) # sample usage
        self.mp,found=[],{}
        if search==None:
            search=[]
        self.node_size,self.edge_size,self.grid_h,self.grid_w=h*w,0,h,w
        self.edges=[[] for i in range(self.node_size)]
        self.IsGrid=True
        for i in range(h):
            s=input()
            for char in search:
                if char in s:
                    found[char]=(i*w+s.index(char))
                    mp_def[char]=mp_def[replacement_of_found]
            self.mp+=[mp_def[j] for j in s]
        for dd in [1,w]:
            for node in range(self.node_size):
                H,W=node//w,node%w
                nx=node+dd
                if 0<=nx<self.node_size:
                    if nx//w==H or nx%w==W:
                        if self.mp[nx]==0:
                            self.edges[node].append((nx,1))
                        if self.mp[node]==0:
                            self.edges[nx].append((node,1))
        return self.mp,found

    def dfs(self,x,func=lambda pr,prv,nx,dist:prv+dist,root_v=0):
        q=deque([x])
        v=[None]*self.node_size
        v[x]=root_v
        while q:
            c=q.pop()
            for nb,d in self.edges[c]:
                if v[nb]==None:
                    q.append(nb)
                    v[nb]=func(c,v[c],nb,d)
        return v

    def bfs(self,x,func=lambda pr,prv,nx,dist:prv+dist,root_v=0):
        q=deque([x])
        q=deque([i for i,j in enumerate(self.mp) if j==1])
        v=[None]*self.node_size
        #v[x]=root_v
        for y in q:v[y]=root_v
        while q:
            c=q.popleft()
            for nb,d in self.edges[c]:
                if v[nb]==None:
                    q.append(nb)
                    v[nb]=func(c,v[c],nb,d)
        return v

    def parent(self,x):
        return self.bfs(0,func=lambda pr,prv,nx,dist:pr,root_v=-1)

    def topological_sort(self,x):  # return topological sort of the tree
        tps=[]
        q=deque([x])
        v=[None]*self.node_size
        v[x]=0
        while q:
            c=q.popleft()
            tps.append(c)
            for nb,d in self.edges[c]:
                if v[nb]==None:
                    q.append(nb)
                    v[nb]=0
        return tps

    def dijkstra(self,st):
        # edge=[[(v_to,dist_to_v),...],[],...]
        # initialize: def: d=dist(st,i), prev=[previous vertex in minimum path], q[]
        d=[(0 if st==i else inf) for i in range(self.node_size)]
        prev=[0]*self.node_size
        q=[(j,i) for i,j in enumerate(d)]
        heapify(q)
        v=[False]*self.node_size
        # calc
        while q:
            dist,cur=heappop(q)
            if v[cur]:
                continue
            v[cur]=True
            for dst,dist in self.edges[cur]:
                alt=d[cur]+dist
                if alt<d[dst]:
                    d[dst]=alt
                    prev[dst]=cur
                    heappush(q,(alt,dst))
        return d,prev
    
    def __str__(self):
        if self.IsGrid:
            for i in range(self.grid_h):
                print(*self.mp[i*self.grid_w:-~i*self.grid_w])
            return str(str(self.show()))
        else:
            return  str(str(self.show()))

    def show(self):  # hide distance when all distance == 1
        if self.IsGrid:
            rt=[[nd for nd,d in e] for e in self.edges]
            return 'Grid',rt
        else:
            if all([all([d==1 for nd,d in e]) for e in self.edges]):
                rt=[[nd for nd,d in e] for e in self.edges]
                return 'Simplified',rt
            else:
                return self


gr=Graph(init=False)
h,w=LI()

gr.gridin(h,w)
print(0/0)
v=gr.bfs(0)

print(max(v))




Fixed submission:
from collections import deque
h,w=map(int,input().split())
a=[[] for _ in range(h)]
for i in range(h):
    s=input()
    for j in s:
        if j=='.':
            a[i].append(-1)
        else:
            a[i].append(0)

def bfs():
    d=[[0,1],[1,0],[-1,0],[0,-1]]
    q=deque()
    c=0
    for i in range(h):
        for j in range(w):
            if a[i][j]==0:
                q.append((i,j))
    while(len(q)>0):
        cur=q.popleft()
        i,j=cur
        for dd in d:
            di,dj=dd[0],dd[1]
            if (i+di)*(h-1-i-di)>=0 and (j+dj)*(w-1-j-dj)>=0 and a[i+di][j+dj]<0:
                c=a[i][j]+1
                a[i+di][j+dj]=c
                q.append((i+di,j+dj))
    return(c)

print(bfs())

Buggy python submission (verdict: Time Limit Exceeded):
import sys,bisect,string,math,time,functools,random,fractions
from heapq import heappush,heappop,heapify
from collections import deque,defaultdict,Counter
from itertools import permutations,combinations,groupby
rep=range
def Golf():n,*t=map(int,open(0).read().split())
def I():return int(input())
def S_():return input()
def IS():return input().split()
def LS():return [i for i in input().split()]
def MI():return map(int,input().split())
def LI():return [int(i) for i in input().split()]
def LI_():return [int(i)-1 for i in input().split()]
def NI(n):return [int(input()) for i in range(n)]
def NI_(n):return [int(input())-1 for i in range(n)]
def StoLI():return [ord(i)-97 for i in input()]
def ItoS(n):return chr(n+97)
def LtoS(ls):return ''.join([chr(i+97) for i in ls])
def RA():return map(int,open(0).read().split())
def GI(V,E,ls=None,Directed=False,index=1):
    org_inp=[];g=[[] for i in range(V)]
    FromStdin=True if ls==None else False
    for i in range(E):
        if FromStdin:
            inp=LI()
            org_inp.append(inp)
        else:
            inp=ls[i]
        if len(inp)==2:
            a,b=inp;c=1
        else:
            a,b,c=inp
        if index==1:a-=1;b-=1
        aa=(a,c);bb=(b,c);g[a].append(bb)
        if not Directed:g[b].append(aa)
    return g,org_inp
def GGI(h,w,search=None,replacement_of_found='.',mp_def={'#':1,'.':0},boundary=1):
    #h,w,g,sg=GGI(h,w,search=['S','G'],replacement_of_found='.',mp_def={'#':1,'.':0},boundary=1) # sample usage
    mp=[boundary]*(w+2);found={}
    for i in range(h):
        s=input()
        for char in search:
            if char in s:
                found[char]=((i+1)*(w+2)+s.index(char)+1)
                mp_def[char]=mp_def[replacement_of_found]
        mp+=[boundary]+[mp_def[j] for j in s]+[boundary]
    mp+=[boundary]*(w+2)
    return h+2,w+2,mp,found
def TI(n):return GI(n,n-1)
def accum(ls):
    rt=[0]
    for i in ls:rt+=[rt[-1]+i]
    return rt
def bit_combination(n,base=2):
    rt=[]
    for tb in range(base**n):s=[tb//(base**bt)%base for bt in range(n)];rt+=[s]
    return rt
def gcd(x,y):
    if y==0:return x
    if x%y==0:return y
    while x%y!=0:x,y=y,x%y
    return y
def YN(x):print(['NO','YES'][x])
def Yn(x):print(['No','Yes'][x])
def show(*inp,end='\n'):
    if show_flg:print(*inp,end=end)

mo=10**9+7
inf=float('inf')
FourNb=[(-1,0),(1,0),(0,1),(0,-1)];EightNb=[(-1,0),(1,0),(0,1),(0,-1),(1,1),(-1,-1),(1,-1),(-1,1)];compas=dict(zip('WENS',FourNb));cursol=dict(zip('LRUD',FourNb))
l_alp=string.ascii_lowercase
#sys.setrecursionlimit(10**7)
read=sys.stdin.buffer.read
readline=sys.stdin.buffer.readline
input=lambda: sys.stdin.readline().rstrip()

class Tree:
    def __init__(self,inp_size=None,ls=None,init=True,index=1):
        self.LCA_init_stat=False
        self.ETtable=[]
        if init:
            if ls==None:
                self.stdin(inp_size,index=index)
            else:
                self.node_size=len(ls)+1
                self.edges,_=GI(self.node_size,self.node_size-1,ls,index=index)
        return
 
    def stdin(self,inp_size=None,index=1):
        if inp_size==None:
            self.node_size=int(input())
        else:
            self.node_size=inp_size
        self.edges,_=GI(self.node_size,self.node_size-1,index=index)
        return
    
    def listin(self,ls,index=0):
        self.node_size=len(ls)+1
        self.edges,_=GI(self.node_size,self.node_size-1,ls,index=index)
        return
 
    def dfs(self,x,func=lambda pr,prv,nx,dist:prv+dist,root_v=0):
        q=deque([x])
        v=[None]*self.node_size
        v[x]=root_v
        while q:
            c=q.pop()
            for nb,d in self.edges[c]:
                if v[nb]==None:
                    q.append(nb)
                    v[nb]=func(c,v[c],nb,d)
        return v
 
    def bfs(self,x,func=lambda pr,prv,nx,dist:prv+dist,root_v=0):
        q=deque([x])
        v=[None]*self.node_size
        v[x]=root_v
        while q:
            c=q.popleft()
            for nb,d in self.edges[c]:
                if v[nb]==None:
                    q.append(nb)
                    v[nb]=func(c,v[c],nb,d)
        return v
 
    def parent(self,x):
        return self.dfs(0,func=lambda pr,prv,nx,dist:pr,root_v=-1)
 
    def topological_sort(self,x):  # return topological sort of the tree
        tps=[]
        q=deque([x])
        v=[None]*self.node_size
        v[x]=0
        while q:
            c=q.popleft()
            tps.append(c)
            for nb,d in self.edges[c]:
                if v[nb]==None:
                    q.append(nb)
                    v[nb]=0
        return tps
 
    def EulerTour(self,x):
        q=deque()
        q.append(x)
        self.depth=[None]*self.node_size
        self.depth[x]=0
        self.ETtable=[]
        self.ETdepth=[]
        self.ETin=[-1]*self.node_size
        self.ETout=[-1]*self.node_size
        cnt=0
        while q:
            c=q.pop()
            if c<0:
                ce=~c
            else:
                ce=c
                for nb,d in self.edges[ce]:
                    if self.depth[nb]==None:
                        q.append(~ce)
                        q.append(nb)
                        self.depth[nb]=self.depth[ce]+1
            self.ETtable.append(ce)
            self.ETdepth.append(self.depth[ce])
            if self.ETin[ce]==-1:
                self.ETin[ce]=cnt
            else:
                self.ETout[ce]=cnt
            cnt+=1
        return
    
    def LCA_init(self,root):
        self.EulerTour(root)
        self.st=SparseTable(self.ETdepth,init_func=min,init_idl=inf)
        #self.st=SegTree(self.node_size*2-1,self.ETdepth,function=min,ide=inf)
        self.LCA_init_stat=True
        return
    
    def LCA(self,root,x,y):
        if self.LCA_init_stat==False:
            self.LCA_init(root)
        xin,xout=self.ETin[x],self.ETout[x]
        yin,yout=self.ETin[y],self.ETout[y]
        a=min(xin,yin)
        b=max(xout,yout,xin,yin)
        id_of_min_dep_in_et=self.st.query_id(a,b+1)
        return self.ETtable[id_of_min_dep_in_et]
 
    def __str__(self):
        return  str(self.edges)
 
    def show(self):
        if all([all([d==1 for nd,d in e]) for e in self.edges]):
            print( [[nd for nd,d in e] for e in self.edges] )
        else:
            print(self)

class SparseTable: # O(N log N) for init, O(1) for query(l,r)
    def __init__(self,ls,init_func=min,init_idl=float('inf')):
        self.func=init_func
        self.idl=init_idl
        self.size=len(ls)
        self.N0=self.size.bit_length()
        self.table=[ls[:]]
        self.index=[list(range(self.size))]
        self.lg=[0]*(self.size+1)
        
        for i in range(2,self.size+1):
            self.lg[i]=self.lg[i>>1]+1
 
        for i in range(self.N0):
            tmp=[self.func(self.table[i][j],self.table[i][min(j+(1<<i),self.size-1)]) for j in range(self.size)]
            tmp_id=[self.index[i][j] if self.table[i][j]==self.func(self.table[i][j],self.table[i][min(j+(1<<i),self.size-1)]) else self.index[i][min(j+(1<<i),self.size-1)] for j in range(self.size)]
            self.table+=[tmp]
            self.index+=[tmp_id]
    
    # return func of [l,r)
    def query(self,l,r):
        if r>self.size:r=self.size
        #N=(r-l).bit_length()-1
        N=self.lg[r-l]
        return self.func(self.table[N][l],self.table[N][max(0,r-(1<<N))])
    
    # return index of which val[i] = func of v among [l,r)
    def query_id(self,l,r):
        if r>self.size:r=self.size
        #N=(r-l).bit_length()-1
        N=self.lg[r-l]
        a,b=self.index[N][l],self.index[N][max(0,r-(1<<N))]
        if self.table[0][a]==self.func(self.table[N][l],self.table[N][max(0,r-(1<<N))]):
            b=a
        return b
    
    # return boundary index of r such that func({table[i]} =< x , i in [pos,r]
    def right_bound(self,pos,x):
        k=(self.size-pos).bit_length()
        for j in range(k)[::-1]:
            nx=pos+(1<<j)
            if nx<n and self.query(pos,nx+1)<=x:
                pos+=(1<<j)
        return pos
    
    # return boundary index of l such that func({table[i]} =< x , i in [l,pos]
    def left_bound(self,pos,x):
        k=pos.bit_length()
        for j in range(k)[::-1]:
            nx=pos-(1<<j)
            if 0<=nx and self.query(nx,pos+1)<=x:
                pos-=(1<<j)
        return pos
 
    def __str__(self):
        return str(self.table[0])
 
    def print(self):
        for i in self.table:
            print(*i)


show_flg=False
show_flg=True
ans=0


class Graph:
    def __init__(self,inp_node_size=None,inp_edge_size=None,ls=None,init=True,Directed=False,index=1):
        if init:
            self.IsGrid=False
            if ls==None:
                self.stdin(inp_node_size,inp_edge_size,Directed=Directed,index=index)
            else:
                self.listin(ls,Directed=Directed,index=index)
        else:
            self.edges=[]
        return

    def stdin(self,inp_node_size=None,inp_edge_size=None,Directed=False,index=1):
        if (inp_node_size,inp_edge_size)==(None,None):
            self.node_size,self.edge_size=LI()
            self.edges,_=GI(self.node_size,self.edge_size,Directed=Directed,index=index)
        else:
            self.node_size,self.edge_size=inp_node_size,inp_edge_size
            self.edges,_=GI(self.node_size,self.edge_size,Directed=Directed,index=index)
        return

    def listin(self,inp_node_size,ls,Directed=False,index=0):
        self.node_size,self.edge_size=inp_node_size,len(ls)
        self.edges,_=GI(self.node_size,self.edge_size,ls,Directed=Directed,index=index)
        return

    def edgein(self,inp_edge,index=0):
        self.node_size,self.edge_size=len(inp_edge),0
        self.edges=[0]*self.node_size
        for i in range(self.node_size):
            self.edges[i]=inp_edge[i][:]
        return

    def gridin(self,h,w,search=None,replacement_of_found='.',mp_def={'#':1,'.':0}):
        #h,w,g,sg=GGI(h,w,search=['S','G'],replacement_of_found='.',mp_def={'#':1,'.':0},boundary=1) # sample usage
        self.mp,found=[],{}
        if search==None:
            search=[]
        self.node_size,self.edge_size,self.grid_h,self.grid_w=h*w,0,h,w
        self.edges=[[] for i in range(self.node_size)]
        self.IsGrid=True
        for i in range(h):
            s=input()
            for char in search:
                if char in s:
                    found[char]=(i*w+s.index(char))
                    mp_def[char]=mp_def[replacement_of_found]
            self.mp+=[mp_def[j] for j in s]
        for dd in [1,w]:
            for node in range(self.node_size):
                H,W=node//w,node%w
                nx=node+dd
                if 0<=nx<self.node_size:
                    if nx//w==H or nx%w==W:
                        if self.mp[nx]==0:
                            self.edges[node].append((nx,1))
                        if self.mp[node]==0:
                            self.edges[nx].append((node,1))
        return self.mp,found

    def dfs(self,x,func=lambda pr,prv,nx,dist:prv+dist,root_v=0):
        q=deque([x])
        v=[None]*self.node_size
        v[x]=root_v
        while q:
            c=q.pop()
            for nb,d in self.edges[c]:
                if v[nb]==None:
                    q.append(nb)
                    v[nb]=func(c,v[c],nb,d)
        return v

    def bfs(self,x,func=lambda pr,prv,nx,dist:prv+dist,root_v=0):
        q=deque([x])
        q=deque([i for i,j in enumerate(self.mp) if j==1])
        v=[None]*self.node_size
        #v[x]=root_v
        for y in q:v[y]=root_v
        while q:
            c=q.popleft()
            for nb,d in self.edges[c]:
                if v[nb]==None:
                    q.append(nb)
                    v[nb]=func(c,v[c],nb,d)
        return v

    def parent(self,x):
        return self.bfs(0,func=lambda pr,prv,nx,dist:pr,root_v=-1)

    def topological_sort(self,x):  # return topological sort of the tree
        tps=[]
        q=deque([x])
        v=[None]*self.node_size
        v[x]=0
        while q:
            c=q.popleft()
            tps.append(c)
            for nb,d in self.edges[c]:
                if v[nb]==None:
                    q.append(nb)
                    v[nb]=0
        return tps

    def dijkstra(self,st):
        # edge=[[(v_to,dist_to_v),...],[],...]
        # initialize: def: d=dist(st,i), prev=[previous vertex in minimum path], q[]
        d=[(0 if st==i else inf) for i in range(self.node_size)]
        prev=[0]*self.node_size
        q=[(j,i) for i,j in enumerate(d)]
        heapify(q)
        v=[False]*self.node_size
        # calc
        while q:
            dist,cur=heappop(q)
            if v[cur]:
                continue
            v[cur]=True
            for dst,dist in self.edges[cur]:
                alt=d[cur]+dist
                if alt<d[dst]:
                    d[dst]=alt
                    prev[dst]=cur
                    heappush(q,(alt,dst))
        return d,prev
    
    def __str__(self):
        if self.IsGrid:
            for i in range(self.grid_h):
                print(*self.mp[i*self.grid_w:-~i*self.grid_w])
            return str(str(self.show()))
        else:
            return  str(str(self.show()))

    def show(self):  # hide distance when all distance == 1
        if self.IsGrid:
            rt=[[nd for nd,d in e] for e in self.edges]
            return 'Grid',rt
        else:
            if all([all([d==1 for nd,d in e]) for e in self.edges]):
                rt=[[nd for nd,d in e] for e in self.edges]
                return 'Simplified',rt
            else:
                return self


gr=Graph(init=False)
h,w=LI()

mp,f=gr.gridin(h,w)
v=gr.bfs(0)

print(max(v))




Fixed submission:
from collections import deque
h,w=map(int,input().split())
a=[[] for _ in range(h)]
for i in range(h):
    s=input()
    for j in s:
        if j=='.':
            a[i].append(-1)
        else:
            a[i].append(0)

def bfs():
    d=[[0,1],[1,0],[-1,0],[0,-1]]
    q=deque()
    c=0
    for i in range(h):
        for j in range(w):
            if a[i][j]==0:
                q.append((i,j))
    while(len(q)>0):
        cur=q.popleft()
        i,j=cur
        for dd in d:
            di,dj=dd[0],dd[1]
            if (i+di)*(h-1-i-di)>=0 and (j+dj)*(w-1-j-dj)>=0 and a[i+di][j+dj]<0:
                c=a[i][j]+1
                a[i+di][j+dj]=c
                q.append((i+di,j+dj))
    return(c)

print(bfs())

Buggy python submission (verdict: Runtime Error):
import sys,bisect,string,math,time,functools,random,fractions
from heapq import heappush,heappop,heapify
from collections import deque,defaultdict,Counter
from itertools import permutations,combinations,groupby
rep=range
def Golf():n,*t=map(int,open(0).read().split())
def I():return int(input())
def S_():return input()
def IS():return input().split()
def LS():return [i for i in input().split()]
def MI():return map(int,input().split())
def LI():return [int(i) for i in input().split()]
def LI_():return [int(i)-1 for i in input().split()]
def NI(n):return [int(input()) for i in range(n)]
def NI_(n):return [int(input())-1 for i in range(n)]
def StoLI():return [ord(i)-97 for i in input()]
def ItoS(n):return chr(n+97)
def LtoS(ls):return ''.join([chr(i+97) for i in ls])
def RA():return map(int,open(0).read().split())
def GI(V,E,ls=None,Directed=False,index=1):
    org_inp=[];g=[[] for i in range(V)]
    FromStdin=True if ls==None else False
    for i in range(E):
        if FromStdin:
            inp=LI()
            org_inp.append(inp)
        else:
            inp=ls[i]
        if len(inp)==2:
            a,b=inp;c=1
        else:
            a,b,c=inp
        if index==1:a-=1;b-=1
        aa=(a,c);bb=(b,c);g[a].append(bb)
        if not Directed:g[b].append(aa)
    return g,org_inp
def GGI(h,w,search=None,replacement_of_found='.',mp_def={'#':1,'.':0},boundary=1):
    #h,w,g,sg=GGI(h,w,search=['S','G'],replacement_of_found='.',mp_def={'#':1,'.':0},boundary=1) # sample usage
    mp=[boundary]*(w+2);found={}
    for i in range(h):
        s=input()
        for char in search:
            if char in s:
                found[char]=((i+1)*(w+2)+s.index(char)+1)
                mp_def[char]=mp_def[replacement_of_found]
        mp+=[boundary]+[mp_def[j] for j in s]+[boundary]
    mp+=[boundary]*(w+2)
    return h+2,w+2,mp,found
def TI(n):return GI(n,n-1)
def accum(ls):
    rt=[0]
    for i in ls:rt+=[rt[-1]+i]
    return rt
def bit_combination(n,base=2):
    rt=[]
    for tb in range(base**n):s=[tb//(base**bt)%base for bt in range(n)];rt+=[s]
    return rt
def gcd(x,y):
    if y==0:return x
    if x%y==0:return y
    while x%y!=0:x,y=y,x%y
    return y
def YN(x):print(['NO','YES'][x])
def Yn(x):print(['No','Yes'][x])
def show(*inp,end='\n'):
    if show_flg:print(*inp,end=end)

mo=10**9+7
inf=float('inf')
FourNb=[(-1,0),(1,0),(0,1),(0,-1)];EightNb=[(-1,0),(1,0),(0,1),(0,-1),(1,1),(-1,-1),(1,-1),(-1,1)];compas=dict(zip('WENS',FourNb));cursol=dict(zip('LRUD',FourNb))
l_alp=string.ascii_lowercase
#sys.setrecursionlimit(10**7)
read=sys.stdin.buffer.read
readline=sys.stdin.buffer.readline
input=lambda: sys.stdin.readline().rstrip()

class Tree:
    def __init__(self,inp_size=None,ls=None,init=True,index=1):
        self.LCA_init_stat=False
        self.ETtable=[]
        if init:
            if ls==None:
                self.stdin(inp_size,index=index)
            else:
                self.node_size=len(ls)+1
                self.edges,_=GI(self.node_size,self.node_size-1,ls,index=index)
        return
 
    def stdin(self,inp_size=None,index=1):
        if inp_size==None:
            self.node_size=int(input())
        else:
            self.node_size=inp_size
        self.edges,_=GI(self.node_size,self.node_size-1,index=index)
        return
    
    def listin(self,ls,index=0):
        self.node_size=len(ls)+1
        self.edges,_=GI(self.node_size,self.node_size-1,ls,index=index)
        return
 
    def dfs(self,x,func=lambda pr,prv,nx,dist:prv+dist,root_v=0):
        q=deque([x])
        v=[None]*self.node_size
        v[x]=root_v
        while q:
            c=q.pop()
            for nb,d in self.edges[c]:
                if v[nb]==None:
                    q.append(nb)
                    v[nb]=func(c,v[c],nb,d)
        return v
 
    def bfs(self,x,func=lambda pr,prv,nx,dist:prv+dist,root_v=0):
        q=deque([x])
        v=[None]*self.node_size
        v[x]=root_v
        while q:
            c=q.popleft()
            for nb,d in self.edges[c]:
                if v[nb]==None:
                    q.append(nb)
                    v[nb]=func(c,v[c],nb,d)
        return v
 
    def parent(self,x):
        return self.dfs(0,func=lambda pr,prv,nx,dist:pr,root_v=-1)
 
    def topological_sort(self,x):  # return topological sort of the tree
        tps=[]
        q=deque([x])
        v=[None]*self.node_size
        v[x]=0
        while q:
            c=q.popleft()
            tps.append(c)
            for nb,d in self.edges[c]:
                if v[nb]==None:
                    q.append(nb)
                    v[nb]=0
        return tps
 
    def EulerTour(self,x):
        q=deque()
        q.append(x)
        self.depth=[None]*self.node_size
        self.depth[x]=0
        self.ETtable=[]
        self.ETdepth=[]
        self.ETin=[-1]*self.node_size
        self.ETout=[-1]*self.node_size
        cnt=0
        while q:
            c=q.pop()
            if c<0:
                ce=~c
            else:
                ce=c
                for nb,d in self.edges[ce]:
                    if self.depth[nb]==None:
                        q.append(~ce)
                        q.append(nb)
                        self.depth[nb]=self.depth[ce]+1
            self.ETtable.append(ce)
            self.ETdepth.append(self.depth[ce])
            if self.ETin[ce]==-1:
                self.ETin[ce]=cnt
            else:
                self.ETout[ce]=cnt
            cnt+=1
        return
    
    def LCA_init(self,root):
        self.EulerTour(root)
        self.st=SparseTable(self.ETdepth,init_func=min,init_idl=inf)
        #self.st=SegTree(self.node_size*2-1,self.ETdepth,function=min,ide=inf)
        self.LCA_init_stat=True
        return
    
    def LCA(self,root,x,y):
        if self.LCA_init_stat==False:
            self.LCA_init(root)
        xin,xout=self.ETin[x],self.ETout[x]
        yin,yout=self.ETin[y],self.ETout[y]
        a=min(xin,yin)
        b=max(xout,yout,xin,yin)
        id_of_min_dep_in_et=self.st.query_id(a,b+1)
        return self.ETtable[id_of_min_dep_in_et]
 
    def __str__(self):
        return  str(self.edges)
 
    def show(self):
        if all([all([d==1 for nd,d in e]) for e in self.edges]):
            print( [[nd for nd,d in e] for e in self.edges] )
        else:
            print(self)

class SparseTable: # O(N log N) for init, O(1) for query(l,r)
    def __init__(self,ls,init_func=min,init_idl=float('inf')):
        self.func=init_func
        self.idl=init_idl
        self.size=len(ls)
        self.N0=self.size.bit_length()
        self.table=[ls[:]]
        self.index=[list(range(self.size))]
        self.lg=[0]*(self.size+1)
        
        for i in range(2,self.size+1):
            self.lg[i]=self.lg[i>>1]+1
 
        for i in range(self.N0):
            tmp=[self.func(self.table[i][j],self.table[i][min(j+(1<<i),self.size-1)]) for j in range(self.size)]
            tmp_id=[self.index[i][j] if self.table[i][j]==self.func(self.table[i][j],self.table[i][min(j+(1<<i),self.size-1)]) else self.index[i][min(j+(1<<i),self.size-1)] for j in range(self.size)]
            self.table+=[tmp]
            self.index+=[tmp_id]
    
    # return func of [l,r)
    def query(self,l,r):
        if r>self.size:r=self.size
        #N=(r-l).bit_length()-1
        N=self.lg[r-l]
        return self.func(self.table[N][l],self.table[N][max(0,r-(1<<N))])
    
    # return index of which val[i] = func of v among [l,r)
    def query_id(self,l,r):
        if r>self.size:r=self.size
        #N=(r-l).bit_length()-1
        N=self.lg[r-l]
        a,b=self.index[N][l],self.index[N][max(0,r-(1<<N))]
        if self.table[0][a]==self.func(self.table[N][l],self.table[N][max(0,r-(1<<N))]):
            b=a
        return b
    
    # return boundary index of r such that func({table[i]} =< x , i in [pos,r]
    def right_bound(self,pos,x):
        k=(self.size-pos).bit_length()
        for j in range(k)[::-1]:
            nx=pos+(1<<j)
            if nx<n and self.query(pos,nx+1)<=x:
                pos+=(1<<j)
        return pos
    
    # return boundary index of l such that func({table[i]} =< x , i in [l,pos]
    def left_bound(self,pos,x):
        k=pos.bit_length()
        for j in range(k)[::-1]:
            nx=pos-(1<<j)
            if 0<=nx and self.query(nx,pos+1)<=x:
                pos-=(1<<j)
        return pos
 
    def __str__(self):
        return str(self.table[0])
 
    def print(self):
        for i in self.table:
            print(*i)


show_flg=False
show_flg=True
ans=0


class Graph:
    def __init__(self,inp_node_size=None,inp_edge_size=None,ls=None,init=True,Directed=False,index=1):
        if init:
            self.IsGrid=False
            if ls==None:
                self.stdin(inp_node_size,inp_edge_size,Directed=Directed,index=index)
            else:
                self.listin(ls,Directed=Directed,index=index)
        else:
            self.edges=[]
        return

    def stdin(self,inp_node_size=None,inp_edge_size=None,Directed=False,index=1):
        if (inp_node_size,inp_edge_size)==(None,None):
            self.node_size,self.edge_size=LI()
            self.edges,_=GI(self.node_size,self.edge_size,Directed=Directed,index=index)
        else:
            self.node_size,self.edge_size=inp_node_size,inp_edge_size
            self.edges,_=GI(self.node_size,self.edge_size,Directed=Directed,index=index)
        return

    def listin(self,inp_node_size,ls,Directed=False,index=0):
        self.node_size,self.edge_size=inp_node_size,len(ls)
        self.edges,_=GI(self.node_size,self.edge_size,ls,Directed=Directed,index=index)
        return

    def edgein(self,inp_edge,index=0):
        self.node_size,self.edge_size=len(inp_edge),0
        self.edges=[0]*self.node_size
        for i in range(self.node_size):
            self.edges[i]=inp_edge[i][:]
        return

    def gridin(self,h,w,search=None,replacement_of_found='.',mp_def={'#':1,'.':0}):
        #h,w,g,sg=GGI(h,w,search=['S','G'],replacement_of_found='.',mp_def={'#':1,'.':0},boundary=1) # sample usage
        self.mp,found=[],{}
        if search==None:
            search=[]
        self.node_size,self.edge_size,self.grid_h,self.grid_w=h*w,0,h,w
        self.edges=[[] for i in range(self.node_size)]
        self.IsGrid=True
        for i in range(h):
            s=input()
            for j in s:
              self.mp.append(mp_def[j])
        return

    def dfs(self,x,func=lambda pr,prv,nx,dist:prv+dist,root_v=0):
        q=deque([x])
        v=[None]*self.node_size
        v[x]=root_v
        while q:
            c=q.pop()
            for nb,d in self.edges[c]:
                if v[nb]==None:
                    q.append(nb)
                    v[nb]=func(c,v[c],nb,d)
        return v

    def bfs(self,x,func=lambda pr,prv,nx,dist:prv+dist,root_v=0):
        q=deque([x])
        q=deque([i for i,j in enumerate(self.mp) if j==1])
        v=[None]*self.node_size
        #v[x]=root_v
        for y in q:v[y]=root_v
        while q:
            c=q.popleft()
            for nb,d in self.edges[c]:
                if v[nb]==None:
                    q.append(nb)
                    v[nb]=func(c,v[c],nb,d)
        return v

    def parent(self,x):
        return self.bfs(0,func=lambda pr,prv,nx,dist:pr,root_v=-1)

    def topological_sort(self,x):  # return topological sort of the tree
        tps=[]
        q=deque([x])
        v=[None]*self.node_size
        v[x]=0
        while q:
            c=q.popleft()
            tps.append(c)
            for nb,d in self.edges[c]:
                if v[nb]==None:
                    q.append(nb)
                    v[nb]=0
        return tps

    def dijkstra(self,st):
        # edge=[[(v_to,dist_to_v),...],[],...]
        # initialize: def: d=dist(st,i), prev=[previous vertex in minimum path], q[]
        d=[(0 if st==i else inf) for i in range(self.node_size)]
        prev=[0]*self.node_size
        q=[(j,i) for i,j in enumerate(d)]
        heapify(q)
        v=[False]*self.node_size
        # calc
        while q:
            dist,cur=heappop(q)
            if v[cur]:
                continue
            v[cur]=True
            for dst,dist in self.edges[cur]:
                alt=d[cur]+dist
                if alt<d[dst]:
                    d[dst]=alt
                    prev[dst]=cur
                    heappush(q,(alt,dst))
        return d,prev
    
    def __str__(self):
        if self.IsGrid:
            for i in range(self.grid_h):
                print(*self.mp[i*self.grid_w:-~i*self.grid_w])
            return str(str(self.show()))
        else:
            return  str(str(self.show()))

    def show(self):  # hide distance when all distance == 1
        if self.IsGrid:
            rt=[[nd for nd,d in e] for e in self.edges]
            return 'Grid',rt
        else:
            if all([all([d==1 for nd,d in e]) for e in self.edges]):
                rt=[[nd for nd,d in e] for e in self.edges]
                return 'Simplified',rt
            else:
                return self


gr=Graph(init=False)
h,w=LI()

gr.gridin(h,w)
v=gr.bfs(0,5)

print(max(v))




Fixed submission:
from collections import deque
h,w=map(int,input().split())
a=[[] for _ in range(h)]
for i in range(h):
    s=input()
    for j in s:
        if j=='.':
            a[i].append(-1)
        else:
            a[i].append(0)

def bfs():
    d=[[0,1],[1,0],[-1,0],[0,-1]]
    q=deque()
    c=0
    for i in range(h):
        for j in range(w):
            if a[i][j]==0:
                q.append((i,j))
    while(len(q)>0):
        cur=q.popleft()
        i,j=cur
        for dd in d:
            di,dj=dd[0],dd[1]
            if (i+di)*(h-1-i-di)>=0 and (j+dj)*(w-1-j-dj)>=0 and a[i+di][j+dj]<0:
                c=a[i][j]+1
                a[i+di][j+dj]=c
                q.append((i+di,j+dj))
    return(c)

print(bfs())

Buggy python submission (verdict: Time Limit Exceeded):
import sys,bisect,string,math,time,functools,random,fractions
from heapq import heappush,heappop,heapify
from collections import deque,defaultdict,Counter
from itertools import permutations,combinations,groupby
rep=range
def Golf():n,*t=map(int,open(0).read().split())
def I():return int(input())
def S_():return input()
def IS():return input().split()
def LS():return [i for i in input().split()]
def MI():return map(int,input().split())
def LI():return [int(i) for i in input().split()]
def LI_():return [int(i)-1 for i in input().split()]
def NI(n):return [int(input()) for i in range(n)]
def NI_(n):return [int(input())-1 for i in range(n)]
def StoLI():return [ord(i)-97 for i in input()]
def ItoS(n):return chr(n+97)
def LtoS(ls):return ''.join([chr(i+97) for i in ls])
def RA():return map(int,open(0).read().split())
def GI(V,E,ls=None,Directed=False,index=1):
    org_inp=[];g=[[] for i in range(V)]
    FromStdin=True if ls==None else False
    for i in range(E):
        if FromStdin:
            inp=LI()
            org_inp.append(inp)
        else:
            inp=ls[i]
        if len(inp)==2:
            a,b=inp;c=1
        else:
            a,b,c=inp
        if index==1:a-=1;b-=1
        aa=(a,c);bb=(b,c);g[a].append(bb)
        if not Directed:g[b].append(aa)
    return g,org_inp
def GGI(h,w,search=None,replacement_of_found='.',mp_def={'#':1,'.':0},boundary=1):
    #h,w,g,sg=GGI(h,w,search=['S','G'],replacement_of_found='.',mp_def={'#':1,'.':0},boundary=1) # sample usage
    mp=[boundary]*(w+2);found={}
    for i in range(h):
        s=input()
        for char in search:
            if char in s:
                found[char]=((i+1)*(w+2)+s.index(char)+1)
                mp_def[char]=mp_def[replacement_of_found]
        mp+=[boundary]+[mp_def[j] for j in s]+[boundary]
    mp+=[boundary]*(w+2)
    return h+2,w+2,mp,found
def TI(n):return GI(n,n-1)
def accum(ls):
    rt=[0]
    for i in ls:rt+=[rt[-1]+i]
    return rt
def bit_combination(n,base=2):
    rt=[]
    for tb in range(base**n):s=[tb//(base**bt)%base for bt in range(n)];rt+=[s]
    return rt
def gcd(x,y):
    if y==0:return x
    if x%y==0:return y
    while x%y!=0:x,y=y,x%y
    return y
def YN(x):print(['NO','YES'][x])
def Yn(x):print(['No','Yes'][x])
def show(*inp,end='\n'):
    if show_flg:print(*inp,end=end)

mo=10**9+7
inf=float('inf')
FourNb=[(-1,0),(1,0),(0,1),(0,-1)];EightNb=[(-1,0),(1,0),(0,1),(0,-1),(1,1),(-1,-1),(1,-1),(-1,1)];compas=dict(zip('WENS',FourNb));cursol=dict(zip('LRUD',FourNb))
l_alp=string.ascii_lowercase
#sys.setrecursionlimit(10**7)
read=sys.stdin.buffer.read
readline=sys.stdin.buffer.readline
input=lambda: sys.stdin.readline().rstrip()

class Tree:
    def __init__(self,inp_size=None,ls=None,init=True,index=1):
        self.LCA_init_stat=False
        self.ETtable=[]
        if init:
            if ls==None:
                self.stdin(inp_size,index=index)
            else:
                self.node_size=len(ls)+1
                self.edges,_=GI(self.node_size,self.node_size-1,ls,index=index)
        return
 
    def stdin(self,inp_size=None,index=1):
        if inp_size==None:
            self.node_size=int(input())
        else:
            self.node_size=inp_size
        self.edges,_=GI(self.node_size,self.node_size-1,index=index)
        return
    
    def listin(self,ls,index=0):
        self.node_size=len(ls)+1
        self.edges,_=GI(self.node_size,self.node_size-1,ls,index=index)
        return
 
    def dfs(self,x,func=lambda pr,prv,nx,dist:prv+dist,root_v=0):
        q=deque([x])
        v=[None]*self.node_size
        v[x]=root_v
        while q:
            c=q.pop()
            for nb,d in self.edges[c]:
                if v[nb]==None:
                    q.append(nb)
                    v[nb]=func(c,v[c],nb,d)
        return v
 
    def bfs(self,x,func=lambda pr,prv,nx,dist:prv+dist,root_v=0):
        q=deque([x])
        v=[None]*self.node_size
        v[x]=root_v
        while q:
            c=q.popleft()
            for nb,d in self.edges[c]:
                if v[nb]==None:
                    q.append(nb)
                    v[nb]=func(c,v[c],nb,d)
        return v
 
    def parent(self,x):
        return self.dfs(0,func=lambda pr,prv,nx,dist:pr,root_v=-1)
 
    def topological_sort(self,x):  # return topological sort of the tree
        tps=[]
        q=deque([x])
        v=[None]*self.node_size
        v[x]=0
        while q:
            c=q.popleft()
            tps.append(c)
            for nb,d in self.edges[c]:
                if v[nb]==None:
                    q.append(nb)
                    v[nb]=0
        return tps
 
    def EulerTour(self,x):
        q=deque()
        q.append(x)
        self.depth=[None]*self.node_size
        self.depth[x]=0
        self.ETtable=[]
        self.ETdepth=[]
        self.ETin=[-1]*self.node_size
        self.ETout=[-1]*self.node_size
        cnt=0
        while q:
            c=q.pop()
            if c<0:
                ce=~c
            else:
                ce=c
                for nb,d in self.edges[ce]:
                    if self.depth[nb]==None:
                        q.append(~ce)
                        q.append(nb)
                        self.depth[nb]=self.depth[ce]+1
            self.ETtable.append(ce)
            self.ETdepth.append(self.depth[ce])
            if self.ETin[ce]==-1:
                self.ETin[ce]=cnt
            else:
                self.ETout[ce]=cnt
            cnt+=1
        return
    
    def LCA_init(self,root):
        self.EulerTour(root)
        self.st=SparseTable(self.ETdepth,init_func=min,init_idl=inf)
        #self.st=SegTree(self.node_size*2-1,self.ETdepth,function=min,ide=inf)
        self.LCA_init_stat=True
        return
    
    def LCA(self,root,x,y):
        if self.LCA_init_stat==False:
            self.LCA_init(root)
        xin,xout=self.ETin[x],self.ETout[x]
        yin,yout=self.ETin[y],self.ETout[y]
        a=min(xin,yin)
        b=max(xout,yout,xin,yin)
        id_of_min_dep_in_et=self.st.query_id(a,b+1)
        return self.ETtable[id_of_min_dep_in_et]
 
    def __str__(self):
        return  str(self.edges)
 
    def show(self):
        if all([all([d==1 for nd,d in e]) for e in self.edges]):
            print( [[nd for nd,d in e] for e in self.edges] )
        else:
            print(self)

class SparseTable: # O(N log N) for init, O(1) for query(l,r)
    def __init__(self,ls,init_func=min,init_idl=float('inf')):
        self.func=init_func
        self.idl=init_idl
        self.size=len(ls)
        self.N0=self.size.bit_length()
        self.table=[ls[:]]
        self.index=[list(range(self.size))]
        self.lg=[0]*(self.size+1)
        
        for i in range(2,self.size+1):
            self.lg[i]=self.lg[i>>1]+1
 
        for i in range(self.N0):
            tmp=[self.func(self.table[i][j],self.table[i][min(j+(1<<i),self.size-1)]) for j in range(self.size)]
            tmp_id=[self.index[i][j] if self.table[i][j]==self.func(self.table[i][j],self.table[i][min(j+(1<<i),self.size-1)]) else self.index[i][min(j+(1<<i),self.size-1)] for j in range(self.size)]
            self.table+=[tmp]
            self.index+=[tmp_id]
    
    # return func of [l,r)
    def query(self,l,r):
        if r>self.size:r=self.size
        #N=(r-l).bit_length()-1
        N=self.lg[r-l]
        return self.func(self.table[N][l],self.table[N][max(0,r-(1<<N))])
    
    # return index of which val[i] = func of v among [l,r)
    def query_id(self,l,r):
        if r>self.size:r=self.size
        #N=(r-l).bit_length()-1
        N=self.lg[r-l]
        a,b=self.index[N][l],self.index[N][max(0,r-(1<<N))]
        if self.table[0][a]==self.func(self.table[N][l],self.table[N][max(0,r-(1<<N))]):
            b=a
        return b
    
    # return boundary index of r such that func({table[i]} =< x , i in [pos,r]
    def right_bound(self,pos,x):
        k=(self.size-pos).bit_length()
        for j in range(k)[::-1]:
            nx=pos+(1<<j)
            if nx<n and self.query(pos,nx+1)<=x:
                pos+=(1<<j)
        return pos
    
    # return boundary index of l such that func({table[i]} =< x , i in [l,pos]
    def left_bound(self,pos,x):
        k=pos.bit_length()
        for j in range(k)[::-1]:
            nx=pos-(1<<j)
            if 0<=nx and self.query(nx,pos+1)<=x:
                pos-=(1<<j)
        return pos
 
    def __str__(self):
        return str(self.table[0])
 
    def print(self):
        for i in self.table:
            print(*i)


show_flg=False
show_flg=True
ans=0


class Graph:
    def __init__(self,inp_node_size=None,inp_edge_size=None,ls=None,init=True,Directed=False,index=1):
        if init:
            self.IsGrid=False
            if ls==None:
                self.stdin(inp_node_size,inp_edge_size,Directed=Directed,index=index)
            else:
                self.listin(ls,Directed=Directed,index=index)
        else:
            self.edges=[]
        return

    def stdin(self,inp_node_size=None,inp_edge_size=None,Directed=False,index=1):
        if (inp_node_size,inp_edge_size)==(None,None):
            self.node_size,self.edge_size=LI()
            self.edges,_=GI(self.node_size,self.edge_size,Directed=Directed,index=index)
        else:
            self.node_size,self.edge_size=inp_node_size,inp_edge_size
            self.edges,_=GI(self.node_size,self.edge_size,Directed=Directed,index=index)
        return

    def listin(self,inp_node_size,ls,Directed=False,index=0):
        self.node_size,self.edge_size=inp_node_size,len(ls)
        self.edges,_=GI(self.node_size,self.edge_size,ls,Directed=Directed,index=index)
        return

    def edgein(self,inp_edge,index=0):
        self.node_size,self.edge_size=len(inp_edge),0
        self.edges=[0]*self.node_size
        for i in range(self.node_size):
            self.edges[i]=inp_edge[i][:]
        return

    def gridin(self,h,w,search=None,replacement_of_found='.',mp_def={'#':1,'.':0}):
        #h,w,g,sg=GGI(h,w,search=['S','G'],replacement_of_found='.',mp_def={'#':1,'.':0},boundary=1) # sample usage
        self.mp,found=[],{}
        if search==None:
            search=[]
        self.node_size,self.edge_size,self.grid_h,self.grid_w=h*w,0,h,w
        self.edges=[[] for i in range(self.node_size)]
        self.IsGrid=True
        for i in range(h):
            s=input()
            
            self.mp+=[mp_def[j] for j in s]
        for node in range(self.node_size):
            H,W=node//w,node%w
            for dd in [1,-1,w,-w]:
                nx=node+dd
                if 0<=nx<self.node_size:
                    if self.mp[nx]==0 and (nx//w==H or nx%w==W):
                        self.edges[node].append((nx,1))
        return self.mp,found

    def dfs(self,x,func=lambda pr,prv,nx,dist:prv+dist,root_v=0):
        q=deque([x])
        v=[None]*self.node_size
        v[x]=root_v
        while q:
            c=q.pop()
            for nb,d in self.edges[c]:
                if v[nb]==None:
                    q.append(nb)
                    v[nb]=func(c,v[c],nb,d)
        return v

    def bfs(self,x,func=lambda pr,prv,nx,dist:prv+dist,root_v=0):
        q=deque([x])
        q=deque([i for i,j in enumerate(mp) if j==1])
        v=[None]*self.node_size
        #v[x]=root_v
        for y in q:v[y]=root_v
        while q:
            c=q.popleft()
            for nb,d in self.edges[c]:
                if v[nb]==None:
                    q.append(nb)
                    v[nb]=func(c,v[c],nb,d)
        return v

    def parent(self,x):
        return self.bfs(0,func=lambda pr,prv,nx,dist:pr,root_v=-1)

    def topological_sort(self,x):  # return topological sort of the tree
        tps=[]
        q=deque([x])
        v=[None]*self.node_size
        v[x]=0
        while q:
            c=q.popleft()
            tps.append(c)
            for nb,d in self.edges[c]:
                if v[nb]==None:
                    q.append(nb)
                    v[nb]=0
        return tps

    def dijkstra(self,st):
        # edge=[[(v_to,dist_to_v),...],[],...]
        # initialize: def: d=dist(st,i), prev=[previous vertex in minimum path], q[]
        d=[(0 if st==i else inf) for i in range(self.node_size)]
        prev=[0]*self.node_size
        q=[(j,i) for i,j in enumerate(d)]
        heapify(q)
        v=[False]*self.node_size
        # calc
        while q:
            dist,cur=heappop(q)
            if v[cur]:
                continue
            v[cur]=True
            for dst,dist in self.edges[cur]:
                alt=d[cur]+dist
                if alt<d[dst]:
                    d[dst]=alt
                    prev[dst]=cur
                    heappush(q,(alt,dst))
        return d,prev
    
    def __str__(self):
        if self.IsGrid:
            for i in range(self.grid_h):
                print(*self.mp[i*self.grid_w:-~i*self.grid_w])
            return str(str(self.show()))
        else:
            return  str(str(self.show()))

    def show(self):  # hide distance when all distance == 1
        if self.IsGrid:
            rt=[[nd for nd,d in e] for e in self.edges]
            return 'Grid',rt
        else:
            if all([all([d==1 for nd,d in e]) for e in self.edges]):
                rt=[[nd for nd,d in e] for e in self.edges]
                return 'Simplified',rt
            else:
                return self


gr=Graph(init=False)
h,w=LI()

mp,f=gr.gridin(h,w)

v=gr.bfs(0)

print(max(v))




Fixed submission:
from collections import deque
h,w=map(int,input().split())
a=[[] for _ in range(h)]
for i in range(h):
    s=input()
    for j in s:
        if j=='.':
            a[i].append(-1)
        else:
            a[i].append(0)

def bfs():
    d=[[0,1],[1,0],[-1,0],[0,-1]]
    q=deque()
    c=0
    for i in range(h):
        for j in range(w):
            if a[i][j]==0:
                q.append((i,j))
    while(len(q)>0):
        cur=q.popleft()
        i,j=cur
        for dd in d:
            di,dj=dd[0],dd[1]
            if (i+di)*(h-1-i-di)>=0 and (j+dj)*(w-1-j-dj)>=0 and a[i+di][j+dj]<0:
                c=a[i][j]+1
                a[i+di][j+dj]=c
                q.append((i+di,j+dj))
    return(c)

print(bfs())

Buggy python submission (verdict: Runtime Error):
import sys,bisect,string,math,time,functools,random,fractions
from heapq import heappush,heappop,heapify
from collections import deque,defaultdict,Counter
from itertools import permutations,combinations,groupby
rep=range
def Golf():n,*t=map(int,open(0).read().split())
def I():return int(input())
def S_():return input()
def IS():return input().split()
def LS():return [i for i in input().split()]
def MI():return map(int,input().split())
def LI():return [int(i) for i in input().split()]
def LI_():return [int(i)-1 for i in input().split()]
def NI(n):return [int(input()) for i in range(n)]
def NI_(n):return [int(input())-1 for i in range(n)]
def StoLI():return [ord(i)-97 for i in input()]
def ItoS(n):return chr(n+97)
def LtoS(ls):return ''.join([chr(i+97) for i in ls])
def RA():return map(int,open(0).read().split())
def GI(V,E,ls=None,Directed=False,index=1):
    org_inp=[];g=[[] for i in range(V)]
    FromStdin=True if ls==None else False
    for i in range(E):
        if FromStdin:
            inp=LI()
            org_inp.append(inp)
        else:
            inp=ls[i]
        if len(inp)==2:
            a,b=inp;c=1
        else:
            a,b,c=inp
        if index==1:a-=1;b-=1
        aa=(a,c);bb=(b,c);g[a].append(bb)
        if not Directed:g[b].append(aa)
    return g,org_inp
def GGI(h,w,search=None,replacement_of_found='.',mp_def={'#':1,'.':0},boundary=1):
    #h,w,g,sg=GGI(h,w,search=['S','G'],replacement_of_found='.',mp_def={'#':1,'.':0},boundary=1) # sample usage
    mp=[boundary]*(w+2);found={}
    for i in range(h):
        s=input()
        for char in search:
            if char in s:
                found[char]=((i+1)*(w+2)+s.index(char)+1)
                mp_def[char]=mp_def[replacement_of_found]
        mp+=[boundary]+[mp_def[j] for j in s]+[boundary]
    mp+=[boundary]*(w+2)
    return h+2,w+2,mp,found
def TI(n):return GI(n,n-1)
def accum(ls):
    rt=[0]
    for i in ls:rt+=[rt[-1]+i]
    return rt
def bit_combination(n,base=2):
    rt=[]
    for tb in range(base**n):s=[tb//(base**bt)%base for bt in range(n)];rt+=[s]
    return rt
def gcd(x,y):
    if y==0:return x
    if x%y==0:return y
    while x%y!=0:x,y=y,x%y
    return y
def YN(x):print(['NO','YES'][x])
def Yn(x):print(['No','Yes'][x])
def show(*inp,end='\n'):
    if show_flg:print(*inp,end=end)

mo=10**9+7
inf=float('inf')
FourNb=[(-1,0),(1,0),(0,1),(0,-1)];EightNb=[(-1,0),(1,0),(0,1),(0,-1),(1,1),(-1,-1),(1,-1),(-1,1)];compas=dict(zip('WENS',FourNb));cursol=dict(zip('LRUD',FourNb))
l_alp=string.ascii_lowercase
#sys.setrecursionlimit(10**7)
read=sys.stdin.buffer.read
readline=sys.stdin.buffer.readline
input=lambda: sys.stdin.readline().rstrip()

class Tree:
    def __init__(self,inp_size=None,ls=None,init=True,index=1):
        self.LCA_init_stat=False
        self.ETtable=[]
        if init:
            if ls==None:
                self.stdin(inp_size,index=index)
            else:
                self.node_size=len(ls)+1
                self.edges,_=GI(self.node_size,self.node_size-1,ls,index=index)
        return
 
    def stdin(self,inp_size=None,index=1):
        if inp_size==None:
            self.node_size=int(input())
        else:
            self.node_size=inp_size
        self.edges,_=GI(self.node_size,self.node_size-1,index=index)
        return
    
    def listin(self,ls,index=0):
        self.node_size=len(ls)+1
        self.edges,_=GI(self.node_size,self.node_size-1,ls,index=index)
        return
 
    def dfs(self,x,func=lambda pr,prv,nx,dist:prv+dist,root_v=0):
        q=deque([x])
        v=[None]*self.node_size
        v[x]=root_v
        while q:
            c=q.pop()
            for nb,d in self.edges[c]:
                if v[nb]==None:
                    q.append(nb)
                    v[nb]=func(c,v[c],nb,d)
        return v
 
    def bfs(self,x,func=lambda pr,prv,nx,dist:prv+dist,root_v=0):
        q=deque([x])
        v=[None]*self.node_size
        v[x]=root_v
        while q:
            c=q.popleft()
            for nb,d in self.edges[c]:
                if v[nb]==None:
                    q.append(nb)
                    v[nb]=func(c,v[c],nb,d)
        return v
 
    def parent(self,x):
        return self.dfs(0,func=lambda pr,prv,nx,dist:pr,root_v=-1)
 
    def topological_sort(self,x):  # return topological sort of the tree
        tps=[]
        q=deque([x])
        v=[None]*self.node_size
        v[x]=0
        while q:
            c=q.popleft()
            tps.append(c)
            for nb,d in self.edges[c]:
                if v[nb]==None:
                    q.append(nb)
                    v[nb]=0
        return tps
 
    def EulerTour(self,x):
        q=deque()
        q.append(x)
        self.depth=[None]*self.node_size
        self.depth[x]=0
        self.ETtable=[]
        self.ETdepth=[]
        self.ETin=[-1]*self.node_size
        self.ETout=[-1]*self.node_size
        cnt=0
        while q:
            c=q.pop()
            if c<0:
                ce=~c
            else:
                ce=c
                for nb,d in self.edges[ce]:
                    if self.depth[nb]==None:
                        q.append(~ce)
                        q.append(nb)
                        self.depth[nb]=self.depth[ce]+1
            self.ETtable.append(ce)
            self.ETdepth.append(self.depth[ce])
            if self.ETin[ce]==-1:
                self.ETin[ce]=cnt
            else:
                self.ETout[ce]=cnt
            cnt+=1
        return
    
    def LCA_init(self,root):
        self.EulerTour(root)
        self.st=SparseTable(self.ETdepth,init_func=min,init_idl=inf)
        #self.st=SegTree(self.node_size*2-1,self.ETdepth,function=min,ide=inf)
        self.LCA_init_stat=True
        return
    
    def LCA(self,root,x,y):
        if self.LCA_init_stat==False:
            self.LCA_init(root)
        xin,xout=self.ETin[x],self.ETout[x]
        yin,yout=self.ETin[y],self.ETout[y]
        a=min(xin,yin)
        b=max(xout,yout,xin,yin)
        id_of_min_dep_in_et=self.st.query_id(a,b+1)
        return self.ETtable[id_of_min_dep_in_et]
 
    def __str__(self):
        return  str(self.edges)
 
    def show(self):
        if all([all([d==1 for nd,d in e]) for e in self.edges]):
            print( [[nd for nd,d in e] for e in self.edges] )
        else:
            print(self)

class SparseTable: # O(N log N) for init, O(1) for query(l,r)
    def __init__(self,ls,init_func=min,init_idl=float('inf')):
        self.func=init_func
        self.idl=init_idl
        self.size=len(ls)
        self.N0=self.size.bit_length()
        self.table=[ls[:]]
        self.index=[list(range(self.size))]
        self.lg=[0]*(self.size+1)
        
        for i in range(2,self.size+1):
            self.lg[i]=self.lg[i>>1]+1
 
        for i in range(self.N0):
            tmp=[self.func(self.table[i][j],self.table[i][min(j+(1<<i),self.size-1)]) for j in range(self.size)]
            tmp_id=[self.index[i][j] if self.table[i][j]==self.func(self.table[i][j],self.table[i][min(j+(1<<i),self.size-1)]) else self.index[i][min(j+(1<<i),self.size-1)] for j in range(self.size)]
            self.table+=[tmp]
            self.index+=[tmp_id]
    
    # return func of [l,r)
    def query(self,l,r):
        if r>self.size:r=self.size
        #N=(r-l).bit_length()-1
        N=self.lg[r-l]
        return self.func(self.table[N][l],self.table[N][max(0,r-(1<<N))])
    
    # return index of which val[i] = func of v among [l,r)
    def query_id(self,l,r):
        if r>self.size:r=self.size
        #N=(r-l).bit_length()-1
        N=self.lg[r-l]
        a,b=self.index[N][l],self.index[N][max(0,r-(1<<N))]
        if self.table[0][a]==self.func(self.table[N][l],self.table[N][max(0,r-(1<<N))]):
            b=a
        return b
    
    # return boundary index of r such that func({table[i]} =< x , i in [pos,r]
    def right_bound(self,pos,x):
        k=(self.size-pos).bit_length()
        for j in range(k)[::-1]:
            nx=pos+(1<<j)
            if nx<n and self.query(pos,nx+1)<=x:
                pos+=(1<<j)
        return pos
    
    # return boundary index of l such that func({table[i]} =< x , i in [l,pos]
    def left_bound(self,pos,x):
        k=pos.bit_length()
        for j in range(k)[::-1]:
            nx=pos-(1<<j)
            if 0<=nx and self.query(nx,pos+1)<=x:
                pos-=(1<<j)
        return pos
 
    def __str__(self):
        return str(self.table[0])
 
    def print(self):
        for i in self.table:
            print(*i)


show_flg=False
show_flg=True
ans=0


class Graph:
    def __init__(self,inp_node_size=None,inp_edge_size=None,ls=None,init=True,Directed=False,index=1):
        if init:
            self.IsGrid=False
            if ls==None:
                self.stdin(inp_node_size,inp_edge_size,Directed=Directed,index=index)
            else:
                self.listin(ls,Directed=Directed,index=index)
        else:
            self.edges=[]
        return

    def stdin(self,inp_node_size=None,inp_edge_size=None,Directed=False,index=1):
        if (inp_node_size,inp_edge_size)==(None,None):
            self.node_size,self.edge_size=LI()
            self.edges,_=GI(self.node_size,self.edge_size,Directed=Directed,index=index)
        else:
            self.node_size,self.edge_size=inp_node_size,inp_edge_size
            self.edges,_=GI(self.node_size,self.edge_size,Directed=Directed,index=index)
        return

    def listin(self,inp_node_size,ls,Directed=False,index=0):
        self.node_size,self.edge_size=inp_node_size,len(ls)
        self.edges,_=GI(self.node_size,self.edge_size,ls,Directed=Directed,index=index)
        return

    def edgein(self,inp_edge,index=0):
        self.node_size,self.edge_size=len(inp_edge),0
        self.edges=[0]*self.node_size
        for i in range(self.node_size):
            self.edges[i]=inp_edge[i][:]
        return

    def gridin(self,h,w,search=None,replacement_of_found='.',mp_def={'#':1,'.':0}):
        #h,w,g,sg=GGI(h,w,search=['S','G'],replacement_of_found='.',mp_def={'#':1,'.':0},boundary=1) # sample usage
        self.mp,found=[],{}
        if search==None:
            search=[]
        self.node_size,self.edge_size,self.grid_h,self.grid_w=h*w,0,h,w
        self.edges=[[] for i in range(self.node_size)]
        self.IsGrid=True
        for i in range(h):
            s=input()
            for j in s:
              self.mp.append(mp_def[j])
        for node in range(self.node_size):
            H,W=node//w,node%w
            for dd in [1,-1,w,-w]:
                nx=node+dd
                if 0<=nx<self.node_size:
                    if self.mp[nx]==0 and (nx//w==H or nx%w==W):
                        self.edges[node].append((nx,1))
        return

    def dfs(self,x,func=lambda pr,prv,nx,dist:prv+dist,root_v=0):
        q=deque([x])
        v=[None]*self.node_size
        v[x]=root_v
        while q:
            c=q.pop()
            for nb,d in self.edges[c]:
                if v[nb]==None:
                    q.append(nb)
                    v[nb]=func(c,v[c],nb,d)
        return v

    def bfs(self,x,func=lambda pr,prv,nx,dist:prv+dist,root_v=0):
        q=deque([x])
        q=deque([i for i,j in enumerate(self.mp) if j==1])
        v=[None]*self.node_size
        #v[x]=root_v
        for y in q:v[y]=root_v
        while q:
            c=q.popleft()
            for nb,d in self.edges[c]:
                if v[nb]==None:
                    q.append(nb)
                    v[nb]=func(c,v[c],nb,d)
        return v

    def parent(self,x):
        return self.bfs(0,func=lambda pr,prv,nx,dist:pr,root_v=-1)

    def topological_sort(self,x):  # return topological sort of the tree
        tps=[]
        q=deque([x])
        v=[None]*self.node_size
        v[x]=0
        while q:
            c=q.popleft()
            tps.append(c)
            for nb,d in self.edges[c]:
                if v[nb]==None:
                    q.append(nb)
                    v[nb]=0
        return tps

    def dijkstra(self,st):
        # edge=[[(v_to,dist_to_v),...],[],...]
        # initialize: def: d=dist(st,i), prev=[previous vertex in minimum path], q[]
        d=[(0 if st==i else inf) for i in range(self.node_size)]
        prev=[0]*self.node_size
        q=[(j,i) for i,j in enumerate(d)]
        heapify(q)
        v=[False]*self.node_size
        # calc
        while q:
            dist,cur=heappop(q)
            if v[cur]:
                continue
            v[cur]=True
            for dst,dist in self.edges[cur]:
                alt=d[cur]+dist
                if alt<d[dst]:
                    d[dst]=alt
                    prev[dst]=cur
                    heappush(q,(alt,dst))
        return d,prev
    
    def __str__(self):
        if self.IsGrid:
            for i in range(self.grid_h):
                print(*self.mp[i*self.grid_w:-~i*self.grid_w])
            return str(str(self.show()))
        else:
            return  str(str(self.show()))

    def show(self):  # hide distance when all distance == 1
        if self.IsGrid:
            rt=[[nd for nd,d in e] for e in self.edges]
            return 'Grid',rt
        else:
            if all([all([d==1 for nd,d in e]) for e in self.edges]):
                rt=[[nd for nd,d in e] for e in self.edges]
                return 'Simplified',rt
            else:
                return self


gr=Graph(init=False)
h,w=LI()

gr.gridin(h,w)
v=gr.bfs(0,5)

print(max(v))




Fixed submission:
from collections import deque
h,w=map(int,input().split())
a=[[] for _ in range(h)]
for i in range(h):
    s=input()
    for j in s:
        if j=='.':
            a[i].append(-1)
        else:
            a[i].append(0)

def bfs():
    d=[[0,1],[1,0],[-1,0],[0,-1]]
    q=deque()
    c=0
    for i in range(h):
        for j in range(w):
            if a[i][j]==0:
                q.append((i,j))
    while(len(q)>0):
        cur=q.popleft()
        i,j=cur
        for dd in d:
            di,dj=dd[0],dd[1]
            if (i+di)*(h-1-i-di)>=0 and (j+dj)*(w-1-j-dj)>=0 and a[i+di][j+dj]<0:
                c=a[i][j]+1
                a[i+di][j+dj]=c
                q.append((i+di,j+dj))
    return(c)

print(bfs())

Buggy python submission (verdict: Runtime Error):
import sys,bisect,string,math,time,functools,random,fractions
from heapq import heappush,heappop,heapify
from collections import deque,defaultdict,Counter
from itertools import permutations,combinations,groupby
rep=range
def Golf():n,*t=map(int,open(0).read().split())
def I():return int(input())
def S_():return input()
def IS():return input().split()
def LS():return [i for i in input().split()]
def MI():return map(int,input().split())
def LI():return [int(i) for i in input().split()]
def LI_():return [int(i)-1 for i in input().split()]
def NI(n):return [int(input()) for i in range(n)]
def NI_(n):return [int(input())-1 for i in range(n)]
def StoLI():return [ord(i)-97 for i in input()]
def ItoS(n):return chr(n+97)
def LtoS(ls):return ''.join([chr(i+97) for i in ls])
def RA():return map(int,open(0).read().split())
def GI(V,E,ls=None,Directed=False,index=1):
    org_inp=[];g=[[] for i in range(V)]
    FromStdin=True if ls==None else False
    for i in range(E):
        if FromStdin:
            inp=LI()
            org_inp.append(inp)
        else:
            inp=ls[i]
        if len(inp)==2:
            a,b=inp;c=1
        else:
            a,b,c=inp
        if index==1:a-=1;b-=1
        aa=(a,c);bb=(b,c);g[a].append(bb)
        if not Directed:g[b].append(aa)
    return g,org_inp
def GGI(h,w,search=None,replacement_of_found='.',mp_def={'#':1,'.':0},boundary=1):
    #h,w,g,sg=GGI(h,w,search=['S','G'],replacement_of_found='.',mp_def={'#':1,'.':0},boundary=1) # sample usage
    mp=[boundary]*(w+2);found={}
    for i in range(h):
        s=input()
        for char in search:
            if char in s:
                found[char]=((i+1)*(w+2)+s.index(char)+1)
                mp_def[char]=mp_def[replacement_of_found]
        mp+=[boundary]+[mp_def[j] for j in s]+[boundary]
    mp+=[boundary]*(w+2)
    return h+2,w+2,mp,found
def TI(n):return GI(n,n-1)
def accum(ls):
    rt=[0]
    for i in ls:rt+=[rt[-1]+i]
    return rt
def bit_combination(n,base=2):
    rt=[]
    for tb in range(base**n):s=[tb//(base**bt)%base for bt in range(n)];rt+=[s]
    return rt
def gcd(x,y):
    if y==0:return x
    if x%y==0:return y
    while x%y!=0:x,y=y,x%y
    return y
def YN(x):print(['NO','YES'][x])
def Yn(x):print(['No','Yes'][x])
def show(*inp,end='\n'):
    if show_flg:print(*inp,end=end)

mo=10**9+7
inf=float('inf')
FourNb=[(-1,0),(1,0),(0,1),(0,-1)];EightNb=[(-1,0),(1,0),(0,1),(0,-1),(1,1),(-1,-1),(1,-1),(-1,1)];compas=dict(zip('WENS',FourNb));cursol=dict(zip('LRUD',FourNb))
l_alp=string.ascii_lowercase
#sys.setrecursionlimit(10**7)
read=sys.stdin.buffer.read
readline=sys.stdin.buffer.readline
input=lambda: sys.stdin.readline().rstrip()

class Tree:
    def __init__(self,inp_size=None,ls=None,init=True,index=1):
        self.LCA_init_stat=False
        self.ETtable=[]
        if init:
            if ls==None:
                self.stdin(inp_size,index=index)
            else:
                self.node_size=len(ls)+1
                self.edges,_=GI(self.node_size,self.node_size-1,ls,index=index)
        return
 
    def stdin(self,inp_size=None,index=1):
        if inp_size==None:
            self.node_size=int(input())
        else:
            self.node_size=inp_size
        self.edges,_=GI(self.node_size,self.node_size-1,index=index)
        return
    
    def listin(self,ls,index=0):
        self.node_size=len(ls)+1
        self.edges,_=GI(self.node_size,self.node_size-1,ls,index=index)
        return
 
    def dfs(self,x,func=lambda pr,prv,nx,dist:prv+dist,root_v=0):
        q=deque([x])
        v=[None]*self.node_size
        v[x]=root_v
        while q:
            c=q.pop()
            for nb,d in self.edges[c]:
                if v[nb]==None:
                    q.append(nb)
                    v[nb]=func(c,v[c],nb,d)
        return v
 
    def bfs(self,x,func=lambda pr,prv,nx,dist:prv+dist,root_v=0):
        q=deque([x])
        v=[None]*self.node_size
        v[x]=root_v
        while q:
            c=q.popleft()
            for nb,d in self.edges[c]:
                if v[nb]==None:
                    q.append(nb)
                    v[nb]=func(c,v[c],nb,d)
        return v
 
    def parent(self,x):
        return self.dfs(0,func=lambda pr,prv,nx,dist:pr,root_v=-1)
 
    def topological_sort(self,x):  # return topological sort of the tree
        tps=[]
        q=deque([x])
        v=[None]*self.node_size
        v[x]=0
        while q:
            c=q.popleft()
            tps.append(c)
            for nb,d in self.edges[c]:
                if v[nb]==None:
                    q.append(nb)
                    v[nb]=0
        return tps
 
    def EulerTour(self,x):
        q=deque()
        q.append(x)
        self.depth=[None]*self.node_size
        self.depth[x]=0
        self.ETtable=[]
        self.ETdepth=[]
        self.ETin=[-1]*self.node_size
        self.ETout=[-1]*self.node_size
        cnt=0
        while q:
            c=q.pop()
            if c<0:
                ce=~c
            else:
                ce=c
                for nb,d in self.edges[ce]:
                    if self.depth[nb]==None:
                        q.append(~ce)
                        q.append(nb)
                        self.depth[nb]=self.depth[ce]+1
            self.ETtable.append(ce)
            self.ETdepth.append(self.depth[ce])
            if self.ETin[ce]==-1:
                self.ETin[ce]=cnt
            else:
                self.ETout[ce]=cnt
            cnt+=1
        return
    
    def LCA_init(self,root):
        self.EulerTour(root)
        self.st=SparseTable(self.ETdepth,init_func=min,init_idl=inf)
        #self.st=SegTree(self.node_size*2-1,self.ETdepth,function=min,ide=inf)
        self.LCA_init_stat=True
        return
    
    def LCA(self,root,x,y):
        if self.LCA_init_stat==False:
            self.LCA_init(root)
        xin,xout=self.ETin[x],self.ETout[x]
        yin,yout=self.ETin[y],self.ETout[y]
        a=min(xin,yin)
        b=max(xout,yout,xin,yin)
        id_of_min_dep_in_et=self.st.query_id(a,b+1)
        return self.ETtable[id_of_min_dep_in_et]
 
    def __str__(self):
        return  str(self.edges)
 
    def show(self):
        if all([all([d==1 for nd,d in e]) for e in self.edges]):
            print( [[nd for nd,d in e] for e in self.edges] )
        else:
            print(self)

class SparseTable: # O(N log N) for init, O(1) for query(l,r)
    def __init__(self,ls,init_func=min,init_idl=float('inf')):
        self.func=init_func
        self.idl=init_idl
        self.size=len(ls)
        self.N0=self.size.bit_length()
        self.table=[ls[:]]
        self.index=[list(range(self.size))]
        self.lg=[0]*(self.size+1)
        
        for i in range(2,self.size+1):
            self.lg[i]=self.lg[i>>1]+1
 
        for i in range(self.N0):
            tmp=[self.func(self.table[i][j],self.table[i][min(j+(1<<i),self.size-1)]) for j in range(self.size)]
            tmp_id=[self.index[i][j] if self.table[i][j]==self.func(self.table[i][j],self.table[i][min(j+(1<<i),self.size-1)]) else self.index[i][min(j+(1<<i),self.size-1)] for j in range(self.size)]
            self.table+=[tmp]
            self.index+=[tmp_id]
    
    # return func of [l,r)
    def query(self,l,r):
        if r>self.size:r=self.size
        #N=(r-l).bit_length()-1
        N=self.lg[r-l]
        return self.func(self.table[N][l],self.table[N][max(0,r-(1<<N))])
    
    # return index of which val[i] = func of v among [l,r)
    def query_id(self,l,r):
        if r>self.size:r=self.size
        #N=(r-l).bit_length()-1
        N=self.lg[r-l]
        a,b=self.index[N][l],self.index[N][max(0,r-(1<<N))]
        if self.table[0][a]==self.func(self.table[N][l],self.table[N][max(0,r-(1<<N))]):
            b=a
        return b
    
    # return boundary index of r such that func({table[i]} =< x , i in [pos,r]
    def right_bound(self,pos,x):
        k=(self.size-pos).bit_length()
        for j in range(k)[::-1]:
            nx=pos+(1<<j)
            if nx<n and self.query(pos,nx+1)<=x:
                pos+=(1<<j)
        return pos
    
    # return boundary index of l such that func({table[i]} =< x , i in [l,pos]
    def left_bound(self,pos,x):
        k=pos.bit_length()
        for j in range(k)[::-1]:
            nx=pos-(1<<j)
            if 0<=nx and self.query(nx,pos+1)<=x:
                pos-=(1<<j)
        return pos
 
    def __str__(self):
        return str(self.table[0])
 
    def print(self):
        for i in self.table:
            print(*i)


show_flg=False
show_flg=True
ans=0


class Graph:
    def __init__(self,inp_node_size=None,inp_edge_size=None,ls=None,init=True,Directed=False,index=1):
        if init:
            self.IsGrid=False
            if ls==None:
                self.stdin(inp_node_size,inp_edge_size,Directed=Directed,index=index)
            else:
                self.listin(ls,Directed=Directed,index=index)
        else:
            self.edges=[]
        return

    def stdin(self,inp_node_size=None,inp_edge_size=None,Directed=False,index=1):
        if (inp_node_size,inp_edge_size)==(None,None):
            self.node_size,self.edge_size=LI()
            self.edges,_=GI(self.node_size,self.edge_size,Directed=Directed,index=index)
        else:
            self.node_size,self.edge_size=inp_node_size,inp_edge_size
            self.edges,_=GI(self.node_size,self.edge_size,Directed=Directed,index=index)
        return

    def listin(self,inp_node_size,ls,Directed=False,index=0):
        self.node_size,self.edge_size=inp_node_size,len(ls)
        self.edges,_=GI(self.node_size,self.edge_size,ls,Directed=Directed,index=index)
        return

    def edgein(self,inp_edge,index=0):
        self.node_size,self.edge_size=len(inp_edge),0
        self.edges=[0]*self.node_size
        for i in range(self.node_size):
            self.edges[i]=inp_edge[i][:]
        return

    def gridin(self,h,w,search=None,replacement_of_found='.',mp_def={'#':1,'.':0}):
        #h,w,g,sg=GGI(h,w,search=['S','G'],replacement_of_found='.',mp_def={'#':1,'.':0},boundary=1) # sample usage
        self.mp,found=[],{}
        if search==None:
            search=[]
        self.node_size,self.edge_size,self.grid_h,self.grid_w=h*w,0,h,w
        self.edges=[[] for i in range(self.node_size)]
        self.IsGrid=True
        for i in range(h):
            s=input()
            for j in s:
              self.mp+=mp_def[j],
        for node in range(self.node_size):
            H,W=node//w,node%w
            for dd in [1,-1,w,-w]:
                nx=node+dd
                if 0<=nx<self.node_size:
                    if self.mp[nx]==0 and (nx//w==H or nx%w==W):
                        self.edges[node].append((nx,1))
        return

    def dfs(self,x,func=lambda pr,prv,nx,dist:prv+dist,root_v=0):
        q=deque([x])
        v=[None]*self.node_size
        v[x]=root_v
        while q:
            c=q.pop()
            for nb,d in self.edges[c]:
                if v[nb]==None:
                    q.append(nb)
                    v[nb]=func(c,v[c],nb,d)
        return v

    def bfs(self,x,func=lambda pr,prv,nx,dist:prv+dist,root_v=0):
        q=deque([x])
        q=deque([i for i,j in enumerate(self.mp) if j==1])
        v=[None]*self.node_size
        #v[x]=root_v
        for y in q:v[y]=root_v
        while q:
            c=q.popleft()
            for nb,d in self.edges[c]:
                if v[nb]==None:
                    q.append(nb)
                    v[nb]=func(c,v[c],nb,d)
        return v

    def parent(self,x):
        return self.bfs(0,func=lambda pr,prv,nx,dist:pr,root_v=-1)

    def topological_sort(self,x):  # return topological sort of the tree
        tps=[]
        q=deque([x])
        v=[None]*self.node_size
        v[x]=0
        while q:
            c=q.popleft()
            tps.append(c)
            for nb,d in self.edges[c]:
                if v[nb]==None:
                    q.append(nb)
                    v[nb]=0
        return tps

    def dijkstra(self,st):
        # edge=[[(v_to,dist_to_v),...],[],...]
        # initialize: def: d=dist(st,i), prev=[previous vertex in minimum path], q[]
        d=[(0 if st==i else inf) for i in range(self.node_size)]
        prev=[0]*self.node_size
        q=[(j,i) for i,j in enumerate(d)]
        heapify(q)
        v=[False]*self.node_size
        # calc
        while q:
            dist,cur=heappop(q)
            if v[cur]:
                continue
            v[cur]=True
            for dst,dist in self.edges[cur]:
                alt=d[cur]+dist
                if alt<d[dst]:
                    d[dst]=alt
                    prev[dst]=cur
                    heappush(q,(alt,dst))
        return d,prev
    
    def __str__(self):
        if self.IsGrid:
            for i in range(self.grid_h):
                print(*self.mp[i*self.grid_w:-~i*self.grid_w])
            return str(str(self.show()))
        else:
            return  str(str(self.show()))

    def show(self):  # hide distance when all distance == 1
        if self.IsGrid:
            rt=[[nd for nd,d in e] for e in self.edges]
            return 'Grid',rt
        else:
            if all([all([d==1 for nd,d in e]) for e in self.edges]):
                rt=[[nd for nd,d in e] for e in self.edges]
                return 'Simplified',rt
            else:
                return self


gr=Graph(init=False)
h,w=LI()

gr.gridin(h,w)
v=gr.bfs(0,5)

print(max(v))




Fixed submission:
from collections import deque
h,w=map(int,input().split())
a=[[] for _ in range(h)]
for i in range(h):
    s=input()
    for j in s:
        if j=='.':
            a[i].append(-1)
        else:
            a[i].append(0)

def bfs():
    d=[[0,1],[1,0],[-1,0],[0,-1]]
    q=deque()
    c=0
    for i in range(h):
        for j in range(w):
            if a[i][j]==0:
                q.append((i,j))
    while(len(q)>0):
        cur=q.popleft()
        i,j=cur
        for dd in d:
            di,dj=dd[0],dd[1]
            if (i+di)*(h-1-i-di)>=0 and (j+dj)*(w-1-j-dj)>=0 and a[i+di][j+dj]<0:
                c=a[i][j]+1
                a[i+di][j+dj]=c
                q.append((i+di,j+dj))
    return(c)

print(bfs())

Buggy python submission (verdict: Runtime Error):
import sys,bisect,string,math,time,functools,random,fractions
from heapq import heappush,heappop,heapify
from collections import deque,defaultdict,Counter
from itertools import permutations,combinations,groupby
rep=range
def Golf():n,*t=map(int,open(0).read().split())
def I():return int(input())
def S_():return input()
def IS():return input().split()
def LS():return [i for i in input().split()]
def MI():return map(int,input().split())
def LI():return [int(i) for i in input().split()]
def LI_():return [int(i)-1 for i in input().split()]
def NI(n):return [int(input()) for i in range(n)]
def NI_(n):return [int(input())-1 for i in range(n)]
def StoLI():return [ord(i)-97 for i in input()]
def ItoS(n):return chr(n+97)
def LtoS(ls):return ''.join([chr(i+97) for i in ls])
def RA():return map(int,open(0).read().split())
def GI(V,E,ls=None,Directed=False,index=1):
    org_inp=[];g=[[] for i in range(V)]
    FromStdin=True if ls==None else False
    for i in range(E):
        if FromStdin:
            inp=LI()
            org_inp.append(inp)
        else:
            inp=ls[i]
        if len(inp)==2:
            a,b=inp;c=1
        else:
            a,b,c=inp
        if index==1:a-=1;b-=1
        aa=(a,c);bb=(b,c);g[a].append(bb)
        if not Directed:g[b].append(aa)
    return g,org_inp
def GGI(h,w,search=None,replacement_of_found='.',mp_def={'#':1,'.':0},boundary=1):
    #h,w,g,sg=GGI(h,w,search=['S','G'],replacement_of_found='.',mp_def={'#':1,'.':0},boundary=1) # sample usage
    mp=[boundary]*(w+2);found={}
    for i in range(h):
        s=input()
        for char in search:
            if char in s:
                found[char]=((i+1)*(w+2)+s.index(char)+1)
                mp_def[char]=mp_def[replacement_of_found]
        mp+=[boundary]+[mp_def[j] for j in s]+[boundary]
    mp+=[boundary]*(w+2)
    return h+2,w+2,mp,found
def TI(n):return GI(n,n-1)
def accum(ls):
    rt=[0]
    for i in ls:rt+=[rt[-1]+i]
    return rt
def bit_combination(n,base=2):
    rt=[]
    for tb in range(base**n):s=[tb//(base**bt)%base for bt in range(n)];rt+=[s]
    return rt
def gcd(x,y):
    if y==0:return x
    if x%y==0:return y
    while x%y!=0:x,y=y,x%y
    return y
def YN(x):print(['NO','YES'][x])
def Yn(x):print(['No','Yes'][x])
def show(*inp,end='\n'):
    if show_flg:print(*inp,end=end)

mo=10**9+7
inf=float('inf')
FourNb=[(-1,0),(1,0),(0,1),(0,-1)];EightNb=[(-1,0),(1,0),(0,1),(0,-1),(1,1),(-1,-1),(1,-1),(-1,1)];compas=dict(zip('WENS',FourNb));cursol=dict(zip('LRUD',FourNb))
l_alp=string.ascii_lowercase
#sys.setrecursionlimit(10**7)
read=sys.stdin.buffer.read
readline=sys.stdin.buffer.readline
input=lambda: sys.stdin.readline().rstrip()

class Tree:
    def __init__(self,inp_size=None,ls=None,init=True,index=1):
        self.LCA_init_stat=False
        self.ETtable=[]
        if init:
            if ls==None:
                self.stdin(inp_size,index=index)
            else:
                self.node_size=len(ls)+1
                self.edges,_=GI(self.node_size,self.node_size-1,ls,index=index)
        return
 
    def stdin(self,inp_size=None,index=1):
        if inp_size==None:
            self.node_size=int(input())
        else:
            self.node_size=inp_size
        self.edges,_=GI(self.node_size,self.node_size-1,index=index)
        return
    
    def listin(self,ls,index=0):
        self.node_size=len(ls)+1
        self.edges,_=GI(self.node_size,self.node_size-1,ls,index=index)
        return
 
    def dfs(self,x,func=lambda pr,prv,nx,dist:prv+dist,root_v=0):
        q=deque([x])
        v=[None]*self.node_size
        v[x]=root_v
        while q:
            c=q.pop()
            for nb,d in self.edges[c]:
                if v[nb]==None:
                    q.append(nb)
                    v[nb]=func(c,v[c],nb,d)
        return v
 
    def bfs(self,x,func=lambda pr,prv,nx,dist:prv+dist,root_v=0):
        q=deque([x])
        v=[None]*self.node_size
        v[x]=root_v
        while q:
            c=q.popleft()
            for nb,d in self.edges[c]:
                if v[nb]==None:
                    q.append(nb)
                    v[nb]=func(c,v[c],nb,d)
        return v
 
    def parent(self,x):
        return self.dfs(0,func=lambda pr,prv,nx,dist:pr,root_v=-1)
 
    def topological_sort(self,x):  # return topological sort of the tree
        tps=[]
        q=deque([x])
        v=[None]*self.node_size
        v[x]=0
        while q:
            c=q.popleft()
            tps.append(c)
            for nb,d in self.edges[c]:
                if v[nb]==None:
                    q.append(nb)
                    v[nb]=0
        return tps
 
    def EulerTour(self,x):
        q=deque()
        q.append(x)
        self.depth=[None]*self.node_size
        self.depth[x]=0
        self.ETtable=[]
        self.ETdepth=[]
        self.ETin=[-1]*self.node_size
        self.ETout=[-1]*self.node_size
        cnt=0
        while q:
            c=q.pop()
            if c<0:
                ce=~c
            else:
                ce=c
                for nb,d in self.edges[ce]:
                    if self.depth[nb]==None:
                        q.append(~ce)
                        q.append(nb)
                        self.depth[nb]=self.depth[ce]+1
            self.ETtable.append(ce)
            self.ETdepth.append(self.depth[ce])
            if self.ETin[ce]==-1:
                self.ETin[ce]=cnt
            else:
                self.ETout[ce]=cnt
            cnt+=1
        return
    
    def LCA_init(self,root):
        self.EulerTour(root)
        self.st=SparseTable(self.ETdepth,init_func=min,init_idl=inf)
        #self.st=SegTree(self.node_size*2-1,self.ETdepth,function=min,ide=inf)
        self.LCA_init_stat=True
        return
    
    def LCA(self,root,x,y):
        if self.LCA_init_stat==False:
            self.LCA_init(root)
        xin,xout=self.ETin[x],self.ETout[x]
        yin,yout=self.ETin[y],self.ETout[y]
        a=min(xin,yin)
        b=max(xout,yout,xin,yin)
        id_of_min_dep_in_et=self.st.query_id(a,b+1)
        return self.ETtable[id_of_min_dep_in_et]
 
    def __str__(self):
        return  str(self.edges)
 
    def show(self):
        if all([all([d==1 for nd,d in e]) for e in self.edges]):
            print( [[nd for nd,d in e] for e in self.edges] )
        else:
            print(self)

class SparseTable: # O(N log N) for init, O(1) for query(l,r)
    def __init__(self,ls,init_func=min,init_idl=float('inf')):
        self.func=init_func
        self.idl=init_idl
        self.size=len(ls)
        self.N0=self.size.bit_length()
        self.table=[ls[:]]
        self.index=[list(range(self.size))]
        self.lg=[0]*(self.size+1)
        
        for i in range(2,self.size+1):
            self.lg[i]=self.lg[i>>1]+1
 
        for i in range(self.N0):
            tmp=[self.func(self.table[i][j],self.table[i][min(j+(1<<i),self.size-1)]) for j in range(self.size)]
            tmp_id=[self.index[i][j] if self.table[i][j]==self.func(self.table[i][j],self.table[i][min(j+(1<<i),self.size-1)]) else self.index[i][min(j+(1<<i),self.size-1)] for j in range(self.size)]
            self.table+=[tmp]
            self.index+=[tmp_id]
    
    # return func of [l,r)
    def query(self,l,r):
        if r>self.size:r=self.size
        #N=(r-l).bit_length()-1
        N=self.lg[r-l]
        return self.func(self.table[N][l],self.table[N][max(0,r-(1<<N))])
    
    # return index of which val[i] = func of v among [l,r)
    def query_id(self,l,r):
        if r>self.size:r=self.size
        #N=(r-l).bit_length()-1
        N=self.lg[r-l]
        a,b=self.index[N][l],self.index[N][max(0,r-(1<<N))]
        if self.table[0][a]==self.func(self.table[N][l],self.table[N][max(0,r-(1<<N))]):
            b=a
        return b
    
    # return boundary index of r such that func({table[i]} =< x , i in [pos,r]
    def right_bound(self,pos,x):
        k=(self.size-pos).bit_length()
        for j in range(k)[::-1]:
            nx=pos+(1<<j)
            if nx<n and self.query(pos,nx+1)<=x:
                pos+=(1<<j)
        return pos
    
    # return boundary index of l such that func({table[i]} =< x , i in [l,pos]
    def left_bound(self,pos,x):
        k=pos.bit_length()
        for j in range(k)[::-1]:
            nx=pos-(1<<j)
            if 0<=nx and self.query(nx,pos+1)<=x:
                pos-=(1<<j)
        return pos
 
    def __str__(self):
        return str(self.table[0])
 
    def print(self):
        for i in self.table:
            print(*i)


show_flg=False
show_flg=True
ans=0


class Graph:
    def __init__(self,inp_node_size=None,inp_edge_size=None,ls=None,init=True,Directed=False,index=1):
        if init:
            self.IsGrid=False
            if ls==None:
                self.stdin(inp_node_size,inp_edge_size,Directed=Directed,index=index)
            else:
                self.listin(ls,Directed=Directed,index=index)
        else:
            self.edges=[]
        return

    def stdin(self,inp_node_size=None,inp_edge_size=None,Directed=False,index=1):
        if (inp_node_size,inp_edge_size)==(None,None):
            self.node_size,self.edge_size=LI()
            self.edges,_=GI(self.node_size,self.edge_size,Directed=Directed,index=index)
        else:
            self.node_size,self.edge_size=inp_node_size,inp_edge_size
            self.edges,_=GI(self.node_size,self.edge_size,Directed=Directed,index=index)
        return

    def listin(self,inp_node_size,ls,Directed=False,index=0):
        self.node_size,self.edge_size=inp_node_size,len(ls)
        self.edges,_=GI(self.node_size,self.edge_size,ls,Directed=Directed,index=index)
        return

    def edgein(self,inp_edge,index=0):
        self.node_size,self.edge_size=len(inp_edge),0
        self.edges=[0]*self.node_size
        for i in range(self.node_size):
            self.edges[i]=inp_edge[i][:]
        return

    def gridin(self,h,w,search=None,replacement_of_found='.',mp_def={'#':1,'.':0}):
        #h,w,g,sg=GGI(h,w,search=['S','G'],replacement_of_found='.',mp_def={'#':1,'.':0},boundary=1) # sample usage
        self.mp,found=[],{}
        if search==None:
            search=[]
        self.node_size,self.edge_size,self.grid_h,self.grid_w=h*w,0,h,w
        self.edges=[[] for i in range(self.node_size)]
        self.IsGrid=True
        for i in range(h):
            s=input()
            self.mp+=[mp_def[j] for j in s]
        for node in range(self.node_size):
            H,W=node//w,node%w
            for dd in [1,-1,w,-w]:
                nx=node+dd
                if 0<=nx<self.node_size:
                    if self.mp[nx]==0 and (nx//w==H or nx%w==W):
                        self.edges[node].append((nx,1))
        return

    def dfs(self,x,func=lambda pr,prv,nx,dist:prv+dist,root_v=0):
        q=deque([x])
        v=[None]*self.node_size
        v[x]=root_v
        while q:
            c=q.pop()
            for nb,d in self.edges[c]:
                if v[nb]==None:
                    q.append(nb)
                    v[nb]=func(c,v[c],nb,d)
        return v

    def bfs(self,x,func=lambda pr,prv,nx,dist:prv+dist,root_v=0):
        q=deque([x])
        q=deque([i for i,j in enumerate(self.mp) if j==1])
        v=[None]*self.node_size
        #v[x]=root_v
        for y in q:v[y]=root_v
        while q:
            c=q.popleft()
            for nb,d in self.edges[c]:
                if v[nb]==None:
                    q.append(nb)
                    v[nb]=func(c,v[c],nb,d)
        return v

    def parent(self,x):
        return self.bfs(0,func=lambda pr,prv,nx,dist:pr,root_v=-1)

    def topological_sort(self,x):  # return topological sort of the tree
        tps=[]
        q=deque([x])
        v=[None]*self.node_size
        v[x]=0
        while q:
            c=q.popleft()
            tps.append(c)
            for nb,d in self.edges[c]:
                if v[nb]==None:
                    q.append(nb)
                    v[nb]=0
        return tps

    def dijkstra(self,st):
        # edge=[[(v_to,dist_to_v),...],[],...]
        # initialize: def: d=dist(st,i), prev=[previous vertex in minimum path], q[]
        d=[(0 if st==i else inf) for i in range(self.node_size)]
        prev=[0]*self.node_size
        q=[(j,i) for i,j in enumerate(d)]
        heapify(q)
        v=[False]*self.node_size
        # calc
        while q:
            dist,cur=heappop(q)
            if v[cur]:
                continue
            v[cur]=True
            for dst,dist in self.edges[cur]:
                alt=d[cur]+dist
                if alt<d[dst]:
                    d[dst]=alt
                    prev[dst]=cur
                    heappush(q,(alt,dst))
        return d,prev
    
    def __str__(self):
        if self.IsGrid:
            for i in range(self.grid_h):
                print(*self.mp[i*self.grid_w:-~i*self.grid_w])
            return str(str(self.show()))
        else:
            return  str(str(self.show()))

    def show(self):  # hide distance when all distance == 1
        if self.IsGrid:
            rt=[[nd for nd,d in e] for e in self.edges]
            return 'Grid',rt
        else:
            if all([all([d==1 for nd,d in e]) for e in self.edges]):
                rt=[[nd for nd,d in e] for e in self.edges]
                return 'Simplified',rt
            else:
                return self


gr=Graph(init=False)
h,w=LI()

gr.gridin(h,w)
v=gr.bfs(0,5)

print(max(v))




Fixed submission:
from collections import deque
h,w=map(int,input().split())
a=[[] for _ in range(h)]
for i in range(h):
    s=input()
    for j in s:
        if j=='.':
            a[i].append(-1)
        else:
            a[i].append(0)

def bfs():
    d=[[0,1],[1,0],[-1,0],[0,-1]]
    q=deque()
    c=0
    for i in range(h):
        for j in range(w):
            if a[i][j]==0:
                q.append((i,j))
    while(len(q)>0):
        cur=q.popleft()
        i,j=cur
        for dd in d:
            di,dj=dd[0],dd[1]
            if (i+di)*(h-1-i-di)>=0 and (j+dj)*(w-1-j-dj)>=0 and a[i+di][j+dj]<0:
                c=a[i][j]+1
                a[i+di][j+dj]=c
                q.append((i+di,j+dj))
    return(c)

print(bfs())

Buggy python submission (verdict: Runtime Error):
import sys,bisect,string,math,time,functools,random,fractions
from heapq import heappush,heappop,heapify
from collections import deque,defaultdict,Counter
from itertools import permutations,combinations,groupby
rep=range
def Golf():n,*t=map(int,open(0).read().split())
def I():return int(input())
def S_():return input()
def IS():return input().split()
def LS():return [i for i in input().split()]
def MI():return map(int,input().split())
def LI():return [int(i) for i in input().split()]
def LI_():return [int(i)-1 for i in input().split()]
def NI(n):return [int(input()) for i in range(n)]
def NI_(n):return [int(input())-1 for i in range(n)]
def StoLI():return [ord(i)-97 for i in input()]
def ItoS(n):return chr(n+97)
def LtoS(ls):return ''.join([chr(i+97) for i in ls])
def RA():return map(int,open(0).read().split())
def GI(V,E,ls=None,Directed=False,index=1):
    org_inp=[];g=[[] for i in range(V)]
    FromStdin=True if ls==None else False
    for i in range(E):
        if FromStdin:
            inp=LI()
            org_inp.append(inp)
        else:
            inp=ls[i]
        if len(inp)==2:
            a,b=inp;c=1
        else:
            a,b,c=inp
        if index==1:a-=1;b-=1
        aa=(a,c);bb=(b,c);g[a].append(bb)
        if not Directed:g[b].append(aa)
    return g,org_inp
def GGI(h,w,search=None,replacement_of_found='.',mp_def={'#':1,'.':0},boundary=1):
    #h,w,g,sg=GGI(h,w,search=['S','G'],replacement_of_found='.',mp_def={'#':1,'.':0},boundary=1) # sample usage
    mp=[boundary]*(w+2);found={}
    for i in range(h):
        s=input()
        for char in search:
            if char in s:
                found[char]=((i+1)*(w+2)+s.index(char)+1)
                mp_def[char]=mp_def[replacement_of_found]
        mp+=[boundary]+[mp_def[j] for j in s]+[boundary]
    mp+=[boundary]*(w+2)
    return h+2,w+2,mp,found
def TI(n):return GI(n,n-1)
def accum(ls):
    rt=[0]
    for i in ls:rt+=[rt[-1]+i]
    return rt
def bit_combination(n,base=2):
    rt=[]
    for tb in range(base**n):s=[tb//(base**bt)%base for bt in range(n)];rt+=[s]
    return rt
def gcd(x,y):
    if y==0:return x
    if x%y==0:return y
    while x%y!=0:x,y=y,x%y
    return y
def YN(x):print(['NO','YES'][x])
def Yn(x):print(['No','Yes'][x])
def show(*inp,end='\n'):
    if show_flg:print(*inp,end=end)

mo=10**9+7
inf=float('inf')
FourNb=[(-1,0),(1,0),(0,1),(0,-1)];EightNb=[(-1,0),(1,0),(0,1),(0,-1),(1,1),(-1,-1),(1,-1),(-1,1)];compas=dict(zip('WENS',FourNb));cursol=dict(zip('LRUD',FourNb))
l_alp=string.ascii_lowercase
#sys.setrecursionlimit(10**7)
read=sys.stdin.buffer.read
readline=sys.stdin.buffer.readline
input=lambda: sys.stdin.readline().rstrip()

class Tree:
    def __init__(self,inp_size=None,ls=None,init=True,index=1):
        self.LCA_init_stat=False
        self.ETtable=[]
        if init:
            if ls==None:
                self.stdin(inp_size,index=index)
            else:
                self.node_size=len(ls)+1
                self.edges,_=GI(self.node_size,self.node_size-1,ls,index=index)
        return
 
    def stdin(self,inp_size=None,index=1):
        if inp_size==None:
            self.node_size=int(input())
        else:
            self.node_size=inp_size
        self.edges,_=GI(self.node_size,self.node_size-1,index=index)
        return
    
    def listin(self,ls,index=0):
        self.node_size=len(ls)+1
        self.edges,_=GI(self.node_size,self.node_size-1,ls,index=index)
        return
 
    def dfs(self,x,func=lambda pr,prv,nx,dist:prv+dist,root_v=0):
        q=deque([x])
        v=[None]*self.node_size
        v[x]=root_v
        while q:
            c=q.pop()
            for nb,d in self.edges[c]:
                if v[nb]==None:
                    q.append(nb)
                    v[nb]=func(c,v[c],nb,d)
        return v
 
    def bfs(self,x,func=lambda pr,prv,nx,dist:prv+dist,root_v=0):
        q=deque([x])
        v=[None]*self.node_size
        v[x]=root_v
        while q:
            c=q.popleft()
            for nb,d in self.edges[c]:
                if v[nb]==None:
                    q.append(nb)
                    v[nb]=func(c,v[c],nb,d)
        return v
 
    def parent(self,x):
        return self.dfs(0,func=lambda pr,prv,nx,dist:pr,root_v=-1)
 
    def topological_sort(self,x):  # return topological sort of the tree
        tps=[]
        q=deque([x])
        v=[None]*self.node_size
        v[x]=0
        while q:
            c=q.popleft()
            tps.append(c)
            for nb,d in self.edges[c]:
                if v[nb]==None:
                    q.append(nb)
                    v[nb]=0
        return tps
 
    def EulerTour(self,x):
        q=deque()
        q.append(x)
        self.depth=[None]*self.node_size
        self.depth[x]=0
        self.ETtable=[]
        self.ETdepth=[]
        self.ETin=[-1]*self.node_size
        self.ETout=[-1]*self.node_size
        cnt=0
        while q:
            c=q.pop()
            if c<0:
                ce=~c
            else:
                ce=c
                for nb,d in self.edges[ce]:
                    if self.depth[nb]==None:
                        q.append(~ce)
                        q.append(nb)
                        self.depth[nb]=self.depth[ce]+1
            self.ETtable.append(ce)
            self.ETdepth.append(self.depth[ce])
            if self.ETin[ce]==-1:
                self.ETin[ce]=cnt
            else:
                self.ETout[ce]=cnt
            cnt+=1
        return
    
    def LCA_init(self,root):
        self.EulerTour(root)
        self.st=SparseTable(self.ETdepth,init_func=min,init_idl=inf)
        #self.st=SegTree(self.node_size*2-1,self.ETdepth,function=min,ide=inf)
        self.LCA_init_stat=True
        return
    
    def LCA(self,root,x,y):
        if self.LCA_init_stat==False:
            self.LCA_init(root)
        xin,xout=self.ETin[x],self.ETout[x]
        yin,yout=self.ETin[y],self.ETout[y]
        a=min(xin,yin)
        b=max(xout,yout,xin,yin)
        id_of_min_dep_in_et=self.st.query_id(a,b+1)
        return self.ETtable[id_of_min_dep_in_et]
 
    def __str__(self):
        return  str(self.edges)
 
    def show(self):
        if all([all([d==1 for nd,d in e]) for e in self.edges]):
            print( [[nd for nd,d in e] for e in self.edges] )
        else:
            print(self)

class SparseTable: # O(N log N) for init, O(1) for query(l,r)
    def __init__(self,ls,init_func=min,init_idl=float('inf')):
        self.func=init_func
        self.idl=init_idl
        self.size=len(ls)
        self.N0=self.size.bit_length()
        self.table=[ls[:]]
        self.index=[list(range(self.size))]
        self.lg=[0]*(self.size+1)
        
        for i in range(2,self.size+1):
            self.lg[i]=self.lg[i>>1]+1
 
        for i in range(self.N0):
            tmp=[self.func(self.table[i][j],self.table[i][min(j+(1<<i),self.size-1)]) for j in range(self.size)]
            tmp_id=[self.index[i][j] if self.table[i][j]==self.func(self.table[i][j],self.table[i][min(j+(1<<i),self.size-1)]) else self.index[i][min(j+(1<<i),self.size-1)] for j in range(self.size)]
            self.table+=[tmp]
            self.index+=[tmp_id]
    
    # return func of [l,r)
    def query(self,l,r):
        if r>self.size:r=self.size
        #N=(r-l).bit_length()-1
        N=self.lg[r-l]
        return self.func(self.table[N][l],self.table[N][max(0,r-(1<<N))])
    
    # return index of which val[i] = func of v among [l,r)
    def query_id(self,l,r):
        if r>self.size:r=self.size
        #N=(r-l).bit_length()-1
        N=self.lg[r-l]
        a,b=self.index[N][l],self.index[N][max(0,r-(1<<N))]
        if self.table[0][a]==self.func(self.table[N][l],self.table[N][max(0,r-(1<<N))]):
            b=a
        return b
    
    # return boundary index of r such that func({table[i]} =< x , i in [pos,r]
    def right_bound(self,pos,x):
        k=(self.size-pos).bit_length()
        for j in range(k)[::-1]:
            nx=pos+(1<<j)
            if nx<n and self.query(pos,nx+1)<=x:
                pos+=(1<<j)
        return pos
    
    # return boundary index of l such that func({table[i]} =< x , i in [l,pos]
    def left_bound(self,pos,x):
        k=pos.bit_length()
        for j in range(k)[::-1]:
            nx=pos-(1<<j)
            if 0<=nx and self.query(nx,pos+1)<=x:
                pos-=(1<<j)
        return pos
 
    def __str__(self):
        return str(self.table[0])
 
    def print(self):
        for i in self.table:
            print(*i)


show_flg=False
show_flg=True
ans=0


class Graph:
    def __init__(self,inp_node_size=None,inp_edge_size=None,ls=None,init=True,Directed=False,index=1):
        if init:
            self.IsGrid=False
            if ls==None:
                self.stdin(inp_node_size,inp_edge_size,Directed=Directed,index=index)
            else:
                self.listin(ls,Directed=Directed,index=index)
        else:
            self.edges=[]
        return

    def stdin(self,inp_node_size=None,inp_edge_size=None,Directed=False,index=1):
        if (inp_node_size,inp_edge_size)==(None,None):
            self.node_size,self.edge_size=LI()
            self.edges,_=GI(self.node_size,self.edge_size,Directed=Directed,index=index)
        else:
            self.node_size,self.edge_size=inp_node_size,inp_edge_size
            self.edges,_=GI(self.node_size,self.edge_size,Directed=Directed,index=index)
        return

    def listin(self,inp_node_size,ls,Directed=False,index=0):
        self.node_size,self.edge_size=inp_node_size,len(ls)
        self.edges,_=GI(self.node_size,self.edge_size,ls,Directed=Directed,index=index)
        return

    def edgein(self,inp_edge,index=0):
        self.node_size,self.edge_size=len(inp_edge),0
        self.edges=[0]*self.node_size
        for i in range(self.node_size):
            self.edges[i]=inp_edge[i][:]
        return

    def gridin(self,h,w,search=None,replacement_of_found='.',mp_def={'#':1,'.':0}):
        #h,w,g,sg=GGI(h,w,search=['S','G'],replacement_of_found='.',mp_def={'#':1,'.':0},boundary=1) # sample usage
        self.mp,found=[],{}
        if search==None:
            search=[]
        self.node_size,self.edge_size,self.grid_h,self.grid_w=h*w,0,h,w
        self.edges=[[] for i in range(self.node_size)]
        self.IsGrid=True
        for i in range(h):
            s=input()
            for char in search:
                if char in s:
                    found[char]=(i*w+s.index(char))
                    mp_def[char]=mp_def[replacement_of_found]
            self.mp+=[mp_def[j] for j in s]
        for node in range(self.node_size):
            H,W=node//w,node%w
            for dd in [1,-1,w,-w]:
                nx=node+dd
                if 0<=nx<self.node_size:
                    if self.mp[nx]==0 and (nx//w==H or nx%w==W):
                        self.edges[node].append((nx,1))
        return self.mp,found

    def dfs(self,x,func=lambda pr,prv,nx,dist:prv+dist,root_v=0):
        q=deque([x])
        v=[None]*self.node_size
        v[x]=root_v
        while q:
            c=q.pop()
            for nb,d in self.edges[c]:
                if v[nb]==None:
                    q.append(nb)
                    v[nb]=func(c,v[c],nb,d)
        return v

    def bfs(self,x,func=lambda pr,prv,nx,dist:prv+dist,root_v=0):
        q=deque([x])
        q=deque([i for i,j in enumerate(self.mp) if j==1])
        v=[None]*self.node_size
        #v[x]=root_v
        for y in q:v[y]=root_v
        while q:
            c=q.popleft()
            for nb,d in self.edges[c]:
                if v[nb]==None:
                    q.append(nb)
                    v[nb]=func(c,v[c],nb,d)
        return v

    def parent(self,x):
        return self.bfs(0,func=lambda pr,prv,nx,dist:pr,root_v=-1)

    def topological_sort(self,x):  # return topological sort of the tree
        tps=[]
        q=deque([x])
        v=[None]*self.node_size
        v[x]=0
        while q:
            c=q.popleft()
            tps.append(c)
            for nb,d in self.edges[c]:
                if v[nb]==None:
                    q.append(nb)
                    v[nb]=0
        return tps

    def dijkstra(self,st):
        # edge=[[(v_to,dist_to_v),...],[],...]
        # initialize: def: d=dist(st,i), prev=[previous vertex in minimum path], q[]
        d=[(0 if st==i else inf) for i in range(self.node_size)]
        prev=[0]*self.node_size
        q=[(j,i) for i,j in enumerate(d)]
        heapify(q)
        v=[False]*self.node_size
        # calc
        while q:
            dist,cur=heappop(q)
            if v[cur]:
                continue
            v[cur]=True
            for dst,dist in self.edges[cur]:
                alt=d[cur]+dist
                if alt<d[dst]:
                    d[dst]=alt
                    prev[dst]=cur
                    heappush(q,(alt,dst))
        return d,prev
    
    def __str__(self):
        if self.IsGrid:
            for i in range(self.grid_h):
                print(*self.mp[i*self.grid_w:-~i*self.grid_w])
            return str(str(self.show()))
        else:
            return  str(str(self.show()))

    def show(self):  # hide distance when all distance == 1
        if self.IsGrid:
            rt=[[nd for nd,d in e] for e in self.edges]
            return 'Grid',rt
        else:
            if all([all([d==1 for nd,d in e]) for e in self.edges]):
                rt=[[nd for nd,d in e] for e in self.edges]
                return 'Simplified',rt
            else:
                return self


gr=Graph(init=False)
h,w=LI()

gr.gridin(h,w)
v=gr.bfs(0,5)

print(max(v))




Fixed submission:
from collections import deque
h,w=map(int,input().split())
a=[[] for _ in range(h)]
for i in range(h):
    s=input()
    for j in s:
        if j=='.':
            a[i].append(-1)
        else:
            a[i].append(0)

def bfs():
    d=[[0,1],[1,0],[-1,0],[0,-1]]
    q=deque()
    c=0
    for i in range(h):
        for j in range(w):
            if a[i][j]==0:
                q.append((i,j))
    while(len(q)>0):
        cur=q.popleft()
        i,j=cur
        for dd in d:
            di,dj=dd[0],dd[1]
            if (i+di)*(h-1-i-di)>=0 and (j+dj)*(w-1-j-dj)>=0 and a[i+di][j+dj]<0:
                c=a[i][j]+1
                a[i+di][j+dj]=c
                q.append((i+di,j+dj))
    return(c)

print(bfs())

Buggy python submission (verdict: Runtime Error):
h,w=map(int,input().split())
a=[[] for _ in range(h)]
for i in range(h):
    s=input()
    for j in s:
        if j=='.':
            a[i].append(-1)
        else:
            a[i].append(0)

def bfs():
    d=[[0,1],[1,0],[-1,0],[0,-1]]
    q=[]
    c=-1
    for i in range(h):
        for j in range(w):
            if a[i][j]==0:
                q.append((i,j))
    while q:
        t=q[:]
        q=[]
        c+=1
        for i,j in t:
            for dd in d:
                di,dj=dd[0],dd[1]
                if (i+di)*(h-1-i-di)>=0 and (j+dj)*(w-1-j-dj)>=0 and a[i+di][j+dj]<0:
                    a[i+di][j+dj]=1
                    q.append((i+di,j+dj))
    return(c)

print(bfs(5,5))


Fixed submission:
from collections import deque
h,w=map(int,input().split())
a=[[] for _ in range(h)]
for i in range(h):
    s=input()
    for j in s:
        if j=='.':
            a[i].append(-1)
        else:
            a[i].append(0)

def bfs():
    d=[[0,1],[1,0],[-1,0],[0,-1]]
    q=deque()
    c=0
    for i in range(h):
        for j in range(w):
            if a[i][j]==0:
                q.append((i,j))
    while(len(q)>0):
        cur=q.popleft()
        i,j=cur
        for dd in d:
            di,dj=dd[0],dd[1]
            if (i+di)*(h-1-i-di)>=0 and (j+dj)*(w-1-j-dj)>=0 and a[i+di][j+dj]<0:
                c=a[i][j]+1
                a[i+di][j+dj]=c
                q.append((i+di,j+dj))
    return(c)

print(bfs())