# import sys; input = sys.stdin.buffer.readline
# sys.setrecursionlimit(10**7)
from collections import defaultdict
mod = 10 ** 9 + 7; INF = float("inf")

def getlist():
	return list(map(int, input().split()))

def main():
	H, W = getlist()
	table = []
	for i in range(H):
		l = list(input())
		table.append(l)

	res = 0
	DP = [[0] * W for i in range(H)]
	for i in range(H):
		for j in range(W):
			if table[i][j] == "#":
				DP[i][j] = 1
				res = 1

	for i in range(1, H):
		for j in range(1, W):
			if DP[i][j] == 1:
				DP[i][j] = min(DP[i - 1][j], DP[i][j - 1], DP[i - 1][j - 1]) + 1
			res = max(res, DP[i][j])

	ans = int(res + 1) // 2
	print(ans)

if __name__ == '__main__':
	main()