import sys
input = sys.stdin.readline

H, W = map(int, input().split())
S = []
S.append(["."] * (W + 2))
for i in range(H):
    S.append(["."] + list(input().rstrip()) + ["."])
S.append(["."] * (W + 2))
    
dx = [1, 0, -1, 0, 1, 1, -1, -1]
dy = [0, 1, 0, -1, 1, -1, 1, -1]

from collections import *
Q = deque()
dist = [[-1] * (W + 2) for i in range(H + 2)]
for i in range(H + 2):
    for j in range(W + 2):
        if S[i][j] == ".":
            dist[i][j] = 0
            Q.append((i, j))
            
ans = 0
while Q:
    i, j = Q.popleft()
    for k in range(8):
        x = i + dx[k]
        y = j + dy[k]
        if x < 0 or x > H + 1 or y < 0 or y > W + 1:
            continue
        if dist[x][y] != -1:
            continue
        dist[x][y] = dist[i][j] + 1
        ans = max(ans, dist[x][y])
        Q.append((x, y))
        
print(ans)