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

def bfs():
    q = deque([])
    ans = 0
    
    for i in range(H+2):
        for j in range(W+2):
            if S[i][j]=='.':
                q.append((i, j))
                S[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 S[nx][ny]=='#':
                    S[nx][ny] = S[cx][cy]+1
                    q.append((nx, ny))
                    
                    if ans<S[nx][ny]:
                        ans = S[nx][ny]
    
    return ans

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

print(bfs())