import sys
input = sys.stdin.readline
from collections import deque

def bfs():
    q = deque([])
    dist = [[-1]*(W+2) for _ in range(H+2)]
    
    for i in range(H+2):
        for j in range(W+2):
            if S[i][j]=='.':
                q.append((i, j))
                dist[i][j] = 0
    
    while q:
        cx, cy = q.popleft()
        
        for i in range(-1, 2):
            for j in range(-1, 2):
                if i==0 and j==0:
                    continue
                
                nx, ny = cx+i, cy+j
                
                if 0<=nx<H+2 and 0<=ny<W+2 and dist[nx][ny]==-1:
                    dist[nx][ny] = dist[cx][cy]+1
                    q.append((nx, ny))
    
    return dist

H, W = map(int, input().split())
S = ['.'*(W+2)]+['.'+input()[:-1]+'.' for _ in range(H)]+['.'*(W+2)]
dist = bfs()
ans = 0

for i in range(H+2):
    for j in range(W+2):
        ans = max(ans, dist[i][j])

print(ans)