H, W = map(int, input().split())
S = [input() for _ in range(H)]

dp = [[0] * W for _ in range(H)]
for h in range(H):
    if S[h][0] == '#':
        dp[h][0] = 1
for w in range(1, W):
    if S[0][w] == '#':
        dp[0][w] = 1

M = dp[0][0]
for h in range(1, H):
    for w in range(1, W):
        if S[h][w] == '#':
            dp[h][w] = min(dp[h - 1][w], dp[h][w - 1], dp[h - 1][w - 1]) + 1
            M = max(M, dp[h][w])
print((M + 1) // 2)