H, W = map(int, input().split())
S = [input() for _ in range(H)]
dp = [[1] * W for _ in range(H)]
for h in range(H):
    for w in range(W):
        if S[h][w] == ".":
            dp[h][w] = 0
            continue
        if h > 0:
            if w > 0:
                dp[h][w] = min(dp[h - 1][w], dp[h][w - 1],dp[h-1][w-1]) + 1
            else:
                dp[h][w] = 1
        else:
            dp[h][w] = 1

vmax = 0
for d in dp:
    vmax = max(vmax,max(d))
print(-(-vmax//2))