h, w = map(int, input().split())
S = [input() for _ in range(h)]
cum = [[0] * (w + 1) for _ in range(h + 1)]
for i in range(h):
    for j in range(w):
        if S[i][j] == "#":
            cum[i + 1][j + 1] = 1
        cum[i + 1][j + 1] += cum[i + 1][j] + cum[i][j + 1] - cum[i][j]


def ok(x):
    dp = [[0] * (w + 1) for _ in range(h + 1)]
    for i in range(h - x + 1):
        for j in range(w - x + 1):
            c = cum[i + x][j + x] - cum[i + x][j] - cum[i][j + x] + cum[i][j]
            if c == x * x:
                dp[i][j] += 1
                dp[i + x][j] -= 1
                dp[i][j + x] -= 1
                dp[i + x][j + x] += 1

    for i in range(h):
        for j in range(w - 1):
            dp[i][j + 1] += dp[i][j]

    for i in range(h - 1):
        for j in range(w):
            dp[i + 1][j] += dp[i][j]

    for i in range(h):
        for j in range(w):
            if dp[i][j] == 0 and S[i][j] == "#":
                return False
    return True


l = 1
r = min(h, w) + 1
while r - l > 1:
    mid = (l + r) // 2
    if ok(mid):
        l = mid
    else:
        r = mid

print(l)