import numpy as np
import sys

input = sys.stdin.readline
min1 = np.frompyfunc(lambda x, y: min(x, y) + 1, 2, 1)

H, W = map(int, input().split())
dp = np.where(np.array([list(input()) for _ in range(H)]) == '.', 0, 1)

reset = 1 - dp
reset[:, 0] = 0
for h in range(1, H):
    dp[h, 1:] = np.minimum(dp[h - 1, :-1], dp[h - 1, 1:])
    dp[h, np.where(reset[h])] = -1
    min1.accumulate(dp[h], out=dp[h], dtype=np.object).astype(int)
print((dp.max() + 1) // 2)