Buggy python submission (verdict: Time Limit Exceeded):
import sys
sys.setrecursionlimit(10**7)
from collections import deque

H, W = map(int, input().split())
grid = [input() for i in range(H)]

# 初期の黒('#')の位置からの距離
dist = [[-1]*W for _ in range(H)]

# '#'の位置をキューに追加（＝今回はこれが複数あるので、複数スタートとなる）
black_cells = deque()
for h in range(H):
    for w in range(W):
        if grid[h][w] == '#':
            black_cells.append((h, w))
            dist[h][w] = 0

def bfs(black_cells, dist):
    d = 0
    while black_cells:
        # キューとして使うのでpop()ではなくpopleft()
        h, w = black_cells.popleft()
        d = dist[h][w]
        for dy, dx in ((1, 0), (0, 1), (-1, 0), (0, -1)):
            new_h = h + dy
            new_w = w + dx
            if new_h < 0 or H <= new_h or new_w < 0 or W <= new_w:
                continue
            if dist[new_h][new_w] == -1:  # セルが白('.')であるのと同義
                dist[new_h][new_w] = d+1
                black_cells.append((new_h, new_w))
    return d

d = bfs(black_cells, dist)
print(d)


Fixed submission:
from subprocess import call
call(('pypy3','-c',"""
H,W=map(int,input().split())
A=[list(input()) for i in range(H)]
dy=[0,0,1,-1]
dx=[1,-1,0,0]
m=[[-1]*W for i in range(H)]

from collections import deque
q=deque()
for y in range(H):
  for x in range(W):
    if A[y][x]=='#':
      q.append((y,x))
      m[y][x]=0      
      
def bfs(q,m):
  d=0
  while q:
    y,x=q.popleft()
    d=m[y][x]
    for i in range(4):
      ny=y+dy[i]
      nx=x+dx[i]
      if 0<=ny<H and 0<=nx<W and m[ny][nx]==-1 and A[ny][nx]!='#':
        q.append((ny,nx))
        m[ny][nx]=m[y][x]+1            
bfs(q,m)
ans=0
for i in range(H):
  for j in range(W):
    ans=max(ans,m[i][j])    
print(ans)     
"""))

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

H, W = map(int, input().split())
grid = [input() for i in range(H)]

# 初期の黒('#')の位置からの距離
dist = [[-1]*W for _ in range(H)]

# '#'の位置をキューに追加（＝今回はこれが複数あるので、複数スタートとなる）
black_cells = deque()
for h in range(H):
    for w in range(W):
        if grid[h][w] == '#':
            black_cells.append((h, w))
            dist[h][w] = 0

def bfs(black_cells, dist):
    """
    BFSを用いて、全マス目を黒くする回数を求める

    Args:
        black_cells: 「開始時の黒マス」の座標が入ったキュー（複数座標あり）
        dist: 「開始時の黒マス」からの距離の入ったリスト  [[-1, -1, 0], ...]
        　　　（白マスの初期値は0）

    Returns:
        g: 操作を繰り返した数
    """
    d = 0
    while black_cells:
        # キューとして使うのでpop()ではなくpopleft()
        h, w = black_cells.popleft()
        d = dist[h][w]
        for dy, dx in ((1, 0), (0, 1), (-1, 0), (0, -1)):
            new_h = h + dy
            new_w = w + dx
            if new_h < 0 or H <= new_h or new_w < 0 or W <= new_w:
                continue
            if dist[new_h][new_w] == -1:  # セルが白('.')であるのと同義
                dist[new_h][new_w] = d+1
                black_cells.append((new_h, new_w))
    return d

d = bfs(black_cells, dist)
print(d)

Fixed submission:
from subprocess import call
call(('pypy3','-c',"""
H,W=map(int,input().split())
A=[list(input()) for i in range(H)]
dy=[0,0,1,-1]
dx=[1,-1,0,0]
m=[[-1]*W for i in range(H)]

from collections import deque
q=deque()
for y in range(H):
  for x in range(W):
    if A[y][x]=='#':
      q.append((y,x))
      m[y][x]=0      
      
def bfs(q,m):
  d=0
  while q:
    y,x=q.popleft()
    d=m[y][x]
    for i in range(4):
      ny=y+dy[i]
      nx=x+dx[i]
      if 0<=ny<H and 0<=nx<W and m[ny][nx]==-1 and A[ny][nx]!='#':
        q.append((ny,nx))
        m[ny][nx]=m[y][x]+1            
bfs(q,m)
ans=0
for i in range(H):
  for j in range(W):
    ans=max(ans,m[i][j])    
print(ans)     
"""))

Buggy python submission (verdict: Time Limit Exceeded):
import sys
sys.setrecursionlimit(10**7)
H,W=map(int,input().split())
A=[list(input()) for i in range(H)]
dy=[0,0,1,-1]
dx=[1,-1,0,0]
m=[[-1]*W for i in range(H)]

from collections import deque
q=deque()
for y in range(H):
  for x in range(W):
    if A[y][x]=='#':
      q.append((y,x))
      m[y][x]=0      
      
def bfs(q,m):
  d=0
  while q:
    y,x=q.popleft()
    d=m[y][x]
    for i in range(4):
      ny=y+dy[i]
      nx=x+dx[i]
      if 0<=ny<H and 0<=nx<W and m[ny][nx]==-1 and A[ny][nx]!='#':
        q.append((ny,nx))
        m[ny][nx]=m[y][x]+1            
bfs(q,m)
ans=0
for i in range(H):
  for j in range(W):
    ans=max(ans,m[i][j])    
print(ans) 

Fixed submission:
from subprocess import call
call(('pypy3','-c',"""
H,W=map(int,input().split())
A=[list(input()) for i in range(H)]
dy=[0,0,1,-1]
dx=[1,-1,0,0]
m=[[-1]*W for i in range(H)]

from collections import deque
q=deque()
for y in range(H):
  for x in range(W):
    if A[y][x]=='#':
      q.append((y,x))
      m[y][x]=0      
      
def bfs(q,m):
  d=0
  while q:
    y,x=q.popleft()
    d=m[y][x]
    for i in range(4):
      ny=y+dy[i]
      nx=x+dx[i]
      if 0<=ny<H and 0<=nx<W and m[ny][nx]==-1 and A[ny][nx]!='#':
        q.append((ny,nx))
        m[ny][nx]=m[y][x]+1            
bfs(q,m)
ans=0
for i in range(H):
  for j in range(W):
    ans=max(ans,m[i][j])    
print(ans)     
"""))

Buggy python submission (verdict: Time Limit Exceeded):
import sys
sys.setrecursionlimit(10**7)
H,W=map(int,input().split())
A=[list(input()) for i in range(H)]
INF=10**6
dy=[0,0,1,-1]
dx=[1,-1,0,0]
m=[[INF]*W for i in range(H)]

from collections import deque

q=deque()
for y in range(H):
  for x in range(W):
    if A[y][x]=='#':
      q.append((y,x))
      m[y][x]=0      

      
def bfs(q,m):
  d=0
  while q:
    y,x=q.popleft()
    d=m[y][x]
    for i in range(4):
      ny=y+dy[i]
      nx=x+dx[i]
      if 0<=ny<H and 0<=nx<W and m[ny][nx]==INF and A[ny][nx]!='#':
        q.append((ny,nx))
        m[ny][nx]=m[y][x]+1
            
bfs(q,m)
ans=0
for i in range(H):
  for j in range(W):
    ans=max(ans,m[i][j])
    
print(ans)    

Fixed submission:
from subprocess import call
call(('pypy3','-c',"""
H,W=map(int,input().split())
A=[list(input()) for i in range(H)]
dy=[0,0,1,-1]
dx=[1,-1,0,0]
m=[[-1]*W for i in range(H)]

from collections import deque
q=deque()
for y in range(H):
  for x in range(W):
    if A[y][x]=='#':
      q.append((y,x))
      m[y][x]=0      
      
def bfs(q,m):
  d=0
  while q:
    y,x=q.popleft()
    d=m[y][x]
    for i in range(4):
      ny=y+dy[i]
      nx=x+dx[i]
      if 0<=ny<H and 0<=nx<W and m[ny][nx]==-1 and A[ny][nx]!='#':
        q.append((ny,nx))
        m[ny][nx]=m[y][x]+1            
bfs(q,m)
ans=0
for i in range(H):
  for j in range(W):
    ans=max(ans,m[i][j])    
print(ans)     
"""))

Buggy python submission (verdict: Time Limit Exceeded):
H,W=map(int,input().split())
A=[list(input()) for i in range(H)]
INF=10**6
dy=[0,0,1,-1]
dx=[1,-1,0,0]
m=[[INF]*W for i in range(H)]

from collections import deque

q=deque()
for y in range(H):
  for x in range(W):
    if A[y][x]=='#':
      q.append((y,x))
      m[y][x]=0      

      
def bfs(q,m):
  d=0
  while q:
    y,x=q.popleft()
    d=m[y][x]
    for i in range(4):
      ny=y+dy[i]
      nx=x+dx[i]
      if 0<=ny<H and 0<=nx<W and m[ny][nx]==INF and A[ny][nx]!='#':
        q.append((ny,nx))
        m[ny][nx]=m[y][x]+1
            
bfs(q,m)
ans=0
for i in range(H):
  for j in range(W):
    ans=max(ans,m[i][j])
    
print(ans)     

Fixed submission:
from subprocess import call
call(('pypy3','-c',"""
H,W=map(int,input().split())
A=[list(input()) for i in range(H)]
dy=[0,0,1,-1]
dx=[1,-1,0,0]
m=[[-1]*W for i in range(H)]

from collections import deque
q=deque()
for y in range(H):
  for x in range(W):
    if A[y][x]=='#':
      q.append((y,x))
      m[y][x]=0      
      
def bfs(q,m):
  d=0
  while q:
    y,x=q.popleft()
    d=m[y][x]
    for i in range(4):
      ny=y+dy[i]
      nx=x+dx[i]
      if 0<=ny<H and 0<=nx<W and m[ny][nx]==-1 and A[ny][nx]!='#':
        q.append((ny,nx))
        m[ny][nx]=m[y][x]+1            
bfs(q,m)
ans=0
for i in range(H):
  for j in range(W):
    ans=max(ans,m[i][j])    
print(ans)     
"""))