### BUGGY CODE ###

H,W=map(int,input().split())

l=[]
for i in range(H):
  l.append(list(input()))

if H==1 and W==1:
  print(0)
  exit()

c=0
while True:

  f=True
  for i in range(H):
    if '.' in l[i]:
      f=False

  if f:
    break

  c+=1
  
  for i in range(H):
    for j in range(W):
      x=l[i][j]
      if x=='#':
        if i>0 and i<H-1 and j>0 and j<W-1:
          f1=True
          if l[i-1][j]=='.':
            l[i-1][j]='a'
            f1=False
          if l[i+1][j]=='.':
            l[i+1][j]='a'
            f1=False
          if l[i][j-1]=='.':
            l[i][j-1]='a'
            f1=False
          if l[i][j+1]=='.':
            l[i][j+1]='a'
            f1=False
          if f1:
            l[i][j]='b'
  
        if i==0 and j==0:
          f1=True
          if l[i][j+1]=='.':
            l[i][j+1]='a'
            f1=False
          if l[i+1][j]=='.':
            l[i+1][j]='a'
            f1=False
          if f1:
            l[i][j]='b'
  
        if i==0 and j>0 and j<W-1:
          f1=True
          if l[i+1][j]=='.':
            l[i+1][j]='a'
            f1=False
          if l[i][j-1]=='.':
            l[i][j-1]='a'
            f1=False
          if l[i][j+1]=='.':
            l[i][j+1]='a'
            f1=False
          if f1:
            l[i][j]='b'
  
        if i==0 and j==W-1:
          f1=True
          if l[i][j-1]=='.':
            l[i][j-1]='a'
            f1=False
          if l[i+1][j]=='.':
            l[i+1][j]='a'
            f1=False
          if f1:
            l[i][j]='b'
  
        if i>0 and i<H-1 and j==0:
          f1=True
          if l[i-1][j]=='.':
            l[i-1][j]='a'
            f1=False
          if l[i][j+1]=='.':
            l[i][j+1]='a'
            f1=False
          if l[i+1][j]=='.':
            l[i+1][j]='a'
            f1=False
          if f1:
            l[i][j]='b'
  
        if i>0 and i<H-1 and j==W-1:
          f1=True
          if l[i-1][j]=='.':
            l[i-1][j]='a'
            f1=False
          if l[i][j-1]=='.':
            l[i][j-1]='a'
            f1=False
          if l[i+1][j]=='.':
            l[i+1][j]='a'
            f1=False
          if f1:
            l[i][j]='b'
  
        if i==H-1 and j==0:
          f1=True
          if l[i-1][j]=='.':
            l[i-1][j]='a'
            f1=False
          if l[i][j+1]=='.':
            l[i][j+1]='a'
            f1=False
          if f1:
            l[i][j]='b'

        if i==H-1 and j>0 and j<W-1:
          f1=True
          if l[i-1][j]=='.':
            l[i-1][j]='a'
            f1=False
          if l[i][j-1]=='.':
            l[i][j-1]='a'
            f1=False
          if l[i][j+1]=='.':
            l[i][j+1]='a'
            f1=False
          if f1:
            l[i][j]='b'
  
        if i==H-1 and j==W-1:
          f1=True
          if l[i-1][j]=='.':
            l[i-1][j]='a'
            f1=False
          if l[i][j-1]=='.':
            l[i][j-1]='a'
            f1=False
          if f1:
            l[i][j]='b'
  
    l[i]=list(''.join(l[i]).replace('a','#'))

print(c)

### FIXED CODE ###

import math,itertools,fractions,heapq,collections,bisect,sys,queue,copy

sys.setrecursionlimit(10**7)
inf=10**20
mod=10**9+7
dd=[(-1,0),(0,1),(1,0),(0,-1)]
ddn=[(-1,0),(-1,1),(0,1),(1,1),(1,0),(1,-1),(0,-1),(-1,-1)]

def LI(): return [int(x) for x in sys.stdin.readline().split()]
def I(): return int(sys.stdin.readline())
def LS(): return sys.stdin.readline().split()
def S(): return input()

def main():
  h,w=LI()
  field=[list(S()) for _ in range(h)]
  d=0

  q=collections.deque()
  nokori=h*w
  dist=[[-1]*w for _ in range(h)]
  for i in range(h):
    for j in range(w):
      if field[i][j]=='#':
        q.append((i,j))
        dist[i][j]=0
        nokori-=1

  while q:
    if nokori==0:
      return d

    y,x=q.popleft()

    for dy,dx in dd:
      ny=y+dy
      nx=x+dx

      if 0<=ny and ny<h and 0<=nx and nx<w:
        if field[ny][nx]=='.':
          field[ny][nx]='#'
          d=dist[y][x]+1
          dist[ny][nx]=d
          q.append((ny,nx))
          nokori-=1

  return d

# main()
print(main())