def oi(): return int(input())
def os(): return input()
def mi(): return list(map(int, input().split()))

H, W = mi()
S = [os() for _ in range(H)]
v = [[0 for i in range(H+1)] for j in range(W+1)]
for i in range(1, H+1):
    for j in range(1, W+1):
        if S[i-1][j-1] == '#':
            v[i][j]+=1
        v[i][j] += v[i-1][j] + v[i][j-1] - v[i-1][j-1]

def solve(X):
    w = [[0 for i in range(H+2)] for j in range(W+2)]
    for i in range(1, H-X+2):
        for j in range(1, W-X+2):
            if v[i+X-1][j+X-1]-v[i+X-1][j-1]-v[i-1][j+X-1]+v[i-1][j-1] == X*X:
                w[i][j]+=1
                w[i+X][j]-=1
                w[i][j+X]-=1
                w[i+X][j+X]+=1

    for i in range(0, H+2):
        for j in range(1, W+2):
            w[i][j] += w[i][j-1]

    for i in range(1, H+2):
        for j in range(0, W+2):
            w[i][j] += w[i-1][j];

    for i in range(1, H+1):
        for j in range(1, W+1):
            if S[i-1][j-1] == '#' and w[i][j] == 0:
                return 0
    return 1

l=1
r=min(H, W)+1

while(r-l>1):
    c = (l+r)//2
    if solve(c):
        l=c
    else:
        r=c;

print(l)