from collections import deque
import sys
input = sys.stdin.readline
sys.setrecursionlimit(10 ** 7)


H, W = map(int, input().split())
G = [[0] * (W + 1) for _ in range(H + 1)]
for i in range(1, H + 1):
    for j, v in enumerate(input().rstrip(), 1):
        if v == "#":
            G[i][j] = -1

for i in range(1, H + 1):
    for j in range(1, W + 1):
        if G[i][j] == -1:
            G[i][j] = min(G[i-1][j], G[i][j-1], G[i-1][j-1]) + 1

ans = max(max(g) for g in G)
print((ans + 1) // 2)