#!/usr/bin/env pypy3

import array
import itertools


def main():
    height, width = map(int, input().split())
    square_size = 0
    dp = [array.array("L", (0 for _ in range(width + 2)))
          for _ in range(height + 2)]
    for r in range(1, height + 1):
        row = input()
        for c in range(1, width + 1):
            if row[c - 1] == "#":
                dp[r][c] = min(dp[r - 1][c - 1], dp[r - 1][c], dp[r][c - 1])
                dp[r][c] += 1
                square_size = max(square_size, dp[r][c])
    answer = (square_size + 1) // 2
    print(answer)


if __name__ == '__main__':
    main()