#############################################################

import sys
sys.setrecursionlimit(10**7)

from heapq import heappop,heappush
from collections import deque,defaultdict,Counter
from bisect import bisect_left, bisect_right
from itertools import product,combinations,permutations

ipt = sys.stdin.readline

def iin():
    return int(ipt())
def lmin():
    return list(map(int,ipt().split()))

MOD = 998244353
#############################################################

N,M = lmin()
S = [input() for _ in range(N)]
S = [[0 if S[i][j] == "." else 1 for j in range(M)] for i in range(N)]

T = [[0]*(M+1) for _ in range(N+1)]

for i in range(N):
    for j in range(M):
        T[i+1][j+1] = T[i][j+1]+T[i+1][j]-T[i][j]+S[i][j]

def check(x):
    cnt = [[0]*(M+2) for _ in range(N+2)]
    for i in range(N-x+1):
        for j in range(M-x+1):
            #print(i,j,T[i+x+1][j+x+1]-T[i][j+x+1]-T[i+x+1][j]+T[i][j])
            if T[i+x][j+x]-T[i][j+x]-T[i+x][j]+T[i][j] == x**2:
                cnt[i+1][j+1] += 1
                cnt[i+1][j+x+1] -= 1
                cnt[i+x+1][j+1] -= 1
                cnt[i+x+1][j+x+1] += 1
    
    #print(cnt)
    
    for i in range(1,N+1):
        for j in range(1,M+1):
            cnt[i][j] += cnt[i][j-1]

    for i in range(1,N+1):
        for j in range(1,M+1):
            cnt[i][j] += cnt[i-1][j]


    #print(x,cnt)
    for i in range(1,N+1):
        for j in range(1,N+1):
            if S[i-1][j-1] and not cnt[i][j]:
                return False
    return True            

ok = 1
ng = min(N,M)+1

while ng-ok > 1:
    cen = (ok+ng)//2
    if check(cen):
        ok = cen
    else:
        ng = cen

print(ok)