#yuki_402

h,w=map(int,raw_input().split())
dp=[[0 for i in xrange(w+2)] for j in xrange(h+2)]
res=0
for i in xrange(1,h+1):
 s=raw_input()
 for j in xrange(1,w+1):
  if s[j-1]=='#':
   dp[i][j]=min(dp[i-1][j-1],dp[i-1][j],dp[i][j-1])+1
   res=max(res,dp[i][j])
print (res+1)/2