h, w = map(int, input().split())
S = [list(input()) for _ in range(h)]

ans = 1
DP = [[0 for _ in range(w)] for _ in range(h)]
for i in range(h):
    for j in range(w):
        DP[i][j] = S[i][j] == '#'
for i in range(h - 1):
    for j in range(1, w):
        if DP[i + 1][j] == 1:
            DP[i + 1][j] = min(DP[i + 1][j - 1], DP[i][j], DP[i][j - 1]) + 1
            ans = max(ans, DP[i + 1][j])
print((ans + 1) // 2)