H, W = map(int, input().split())
S = [input() for i in range(H)]
dp = [[0 for i in range(W + 1)] for j in range(H + 1)]
res = 0

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

print((res + 1) // 2)