Buggy python submission (verdict: Time Limit Exceeded):
from collections import deque



def main():
    h,w=map(int,input().split())
    w+=2

    a=[list('@'*w)]
    for i in range(h):
        a+=[list('@'+input()+'@')]
    a+=[list('@'*w)]
    h+=2

    xy=[(-1,0),(0,-1),(0,1),(1,0)]
    que=deque()



    A=[[float('inf')]*w for i in range(h)]

    for i in range(1,h-1):
        for j in range(1,w-1):
            if a[i][j]=='#':
                A[i][j]=0
                que.append((i,j))


    res=0
    while que:
        x,y=que.popleft()
        for dx,dy in xy:
            nx,ny=dx+x,dy+y
            tmp=A[x][y]+1
            if a[nx][ny]!='@':
                if A[nx][ny]<=tmp:continue
                A[nx][ny]=tmp
                que.append((nx,ny))
                res=max(res,tmp)

    print(res)


if __name__ == "__main__":
    main()

Fixed submission:
from collections import deque

h,w=map(int,input().split())
w+=2

a=[list('@'*w)]
for i in range(h):
    a+=[list('@'+input()+'@')]
a+=[list('@'*w)]
h+=2

xy=[(-1,0),(0,-1),(0,1),(1,0)]
que=deque()

def bfs():
    res=0
    while que:
        x,y=que.popleft()
        for dx,dy in xy:
            nx,ny=dx+x,dy+y
            tmp=A[x][y]+1
            if a[nx][ny]!='@':
                if A[nx][ny]<=tmp:continue
                A[nx][ny]=tmp
                que.append((nx,ny))
                res=max(res,tmp)
    return res


A=[[float('inf')]*w for i in range(h)]

res=0
for i in range(1,h-1):
    for j in range(1,w-1):
        if a[i][j]=='#':
            A[i][j]=0
            que.append((i,j))

res=bfs()
print(res)

Buggy python submission (verdict: Time Limit Exceeded):
from collections import deque

h,w=map(int,input().split())
w+=2

a=[list('@'*w)]
for i in range(h):
    a+=[list('@'+input()+'@')]
a+=[list('@'*w)]
h+=2

xy=[(-1,0),(0,-1),(0,1),(1,0)]
que=deque()

def bfs():
    res=0
    while que:
        x,y=que.popleft()
        for dx,dy in xy:
            nx,ny=dx+x,dy+y
            tmp=A[x][y]+1
            if a[nx][ny]!='@':
                if A[nx][ny]<=tmp:continue
                A[nx][ny]=tmp
                que.append((nx,ny))
                res=max(res,tmp)
    return res


A=[[float('inf')]*w for i in range(h)]

res=0
for i in range(1,h-1):
    for j in range(1,w-1):
        if a[i][j]=='#':
            A[i][j]=0
            que.append((i,j))

res=bfs()
print(res)



Fixed submission:
from collections import deque

h,w=map(int,input().split())
w+=2

a=[list('@'*w)]
for i in range(h):
    a+=[list('@'+input()+'@')]
a+=[list('@'*w)]
h+=2

xy=[(-1,0),(0,-1),(0,1),(1,0)]
que=deque()

def bfs():
    res=0
    while que:
        x,y=que.popleft()
        for dx,dy in xy:
            nx,ny=dx+x,dy+y
            tmp=A[x][y]+1
            if a[nx][ny]!='@':
                if A[nx][ny]<=tmp:continue
                A[nx][ny]=tmp
                que.append((nx,ny))
                res=max(res,tmp)
    return res


A=[[float('inf')]*w for i in range(h)]

res=0
for i in range(1,h-1):
    for j in range(1,w-1):
        if a[i][j]=='#':
            A[i][j]=0
            que.append((i,j))

res=bfs()
print(res)

Buggy python submission (verdict: Time Limit Exceeded):
from collections import deque

h,w=map(int,input().split())
w+=2

a=[list('@'*w)]
for i in range(h):
    a+=[list('@'+input()+'@')]
a+=[list('@'*w)]
h+=2

xy=[(-1,0),(0,-1),(0,1),(1,0)]

def bfs(sx,sy):
    A[sx][sy]=0
    que=deque()
    que.append((sx,sy))

    while que:
        x,y=que.popleft()
        for dx,dy in xy:
            nx,ny=dx+x,dy+y
            tmp=A[x][y]+1
            if a[nx][ny]!='@':
                if A[nx][ny]<=tmp:continue
                A[nx][ny]=tmp
                que.append((nx,ny))


A=[[float('inf')]*w for i in range(h)]

res=0
for i in range(1,h-1):
    for j in range(1,w-1):
        if a[i][j]=='#':
            bfs(i,j)

S=A[1:-1]

for e in S:
    res=max(res,max(e[1:-1]))
print(res)


Fixed submission:
from collections import deque

h,w=map(int,input().split())
w+=2

a=[list('@'*w)]
for i in range(h):
    a+=[list('@'+input()+'@')]
a+=[list('@'*w)]
h+=2

xy=[(-1,0),(0,-1),(0,1),(1,0)]
que=deque()

def bfs():
    res=0
    while que:
        x,y=que.popleft()
        for dx,dy in xy:
            nx,ny=dx+x,dy+y
            tmp=A[x][y]+1
            if a[nx][ny]!='@':
                if A[nx][ny]<=tmp:continue
                A[nx][ny]=tmp
                que.append((nx,ny))
                res=max(res,tmp)
    return res


A=[[float('inf')]*w for i in range(h)]

res=0
for i in range(1,h-1):
    for j in range(1,w-1):
        if a[i][j]=='#':
            A[i][j]=0
            que.append((i,j))

res=bfs()
print(res)

Buggy python submission (verdict: Time Limit Exceeded):
from collections import deque

h,w=map(int,input().split())
w+=2

a=[list('@'*w)]
for i in range(h):
    a+=[list('@'+input()+'@')]
a+=[list('@'*w)]
h+=2

xy=[(-1,0),(0,-1),(0,1),(1,0)]

def bfs(sx,sy):
    A[sx][sy]=0
    que=deque()
    que.append((sx,sy))

    while que:
        x,y=que.popleft()
        for dx,dy in xy:
            nx,ny=dx+x,dy+y
            tmp=A[x][y]+1
            if a[nx][ny]!='@':
                if A[nx][ny]<=tmp:continue
                A[nx][ny]=tmp
                que.append((nx,ny))


A=[[float('inf')]*w for i in range(h)]

res=0
for i in range(1,h-1):
    for j in range(1,w-1):
        if a[i][j]=='#':
            bfs(i,j)

S=A[1:-1]

for e in S:
    res=max(res,max(e[1:-1]))
print(res)


Fixed submission:
from collections import deque

h,w=map(int,input().split())
w+=2

a=[list('@'*w)]
for i in range(h):
    a+=[list('@'+input()+'@')]
a+=[list('@'*w)]
h+=2

xy=[(-1,0),(0,-1),(0,1),(1,0)]
que=deque()

def bfs():
    res=0
    while que:
        x,y=que.popleft()
        for dx,dy in xy:
            nx,ny=dx+x,dy+y
            tmp=A[x][y]+1
            if a[nx][ny]!='@':
                if A[nx][ny]<=tmp:continue
                A[nx][ny]=tmp
                que.append((nx,ny))
                res=max(res,tmp)
    return res


A=[[float('inf')]*w for i in range(h)]

res=0
for i in range(1,h-1):
    for j in range(1,w-1):
        if a[i][j]=='#':
            A[i][j]=0
            que.append((i,j))

res=bfs()
print(res)