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

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