# import pypyjit
# pypyjit.set_param('max_unroll_recursion=-1')
from collections import *
from itertools import *
from functools import *
from heapq import *
import sys,math
input = sys.stdin.readline

H,W = map(int,input().split())
M = 1
dp0 = list(input())[:-1]
dp0 = [int(i=='#') for i in dp0]
for _ in range(H-1):
    dp1 = list(input())[:-1]
    dp1 = [int(i=='#') for i in dp1]
    for j in range(1,W):
        if dp1[j]==1:
            dp1[j] = min(dp1[j-1],dp0[j],dp0[j-1])+1
            M = max(M,dp1[j])
    dp0 = dp1[:]
print((M+1)//2)