#!/usr/bin/env pypy3

import array
import itertools


def compute(height, width, is_sea):
    square_size = 0
    dp = [array.array("L", (0 for _ in range(width + 2)))
          for _ in range(height + 2)]
    for r, c in itertools.product(range(1, height + 1), range(1, width + 1)):
        if not is_sea[r - 1][c - 1]:
            dp[r][c] = min(dp[r - 1][c - 1], dp[r - 1][c], dp[r][c - 1]) + 1
            square_size = max(square_size, dp[r][c])
    return (square_size + 1) // 2


def main():
    height, width = map(int, input().split())
    is_sea = [array.array("B", map(lambda x: x == ".", input()))
              for _ in range(height)]
    print(compute(height, width, is_sea))


if __name__ == '__main__':
    main()