import sys
input = sys.stdin.readline

H, W = map(int, input().split())
S = [input().rstrip() for _ in range(H)]

cumsum = [[0] * (W+1) for _ in range(H+1)]
for i in range(H):
    for j in range(W):
        cumsum[i+1][j+1] = cumsum[i][j+1] + cumsum[i+1][j] - \
            cumsum[i][j] + (1 if S[i][j] == '#' else 0)

lb, ub = 0, 2000
while ub-lb > 1:
    m = (lb+ub)//2
    painted = [[0]*(W+1) for _ in range(H+1)]

    for i in range(H-m+1):
        for j in range(W-m+1):
            if cumsum[i+m][j+m]-cumsum[i+m][j]-cumsum[i][j+m]+cumsum[i][j] == m*m:
                painted[i][j] += 1
                painted[i+m][j] -= 1
                painted[i][j+m] -= 1
                painted[i+m][j+m] += 1

    for i in range(H+1):
        for j in range(W):
            painted[i][j+1] += painted[i][j]
    for j in range(W+1):
        for i in range(H):
            painted[i+1][j] += painted[i][j]

    ok = True
    for i in range(H):
        for j in range(W):
            if S[i][j] == '#' and not painted[i][j]:
                ok = False
                break
        if not ok:
            break

    if ok:
        lb = m
    else:
        ub = m

print(lb)