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

H,W=map(int,input().split())
MAP=[input() for i in range(H)]

ANS=[[1<<30]*(W+2) for i in range(H+2)]

Q=deque()

for i in range(H+2):
    ANS[i][0]=0
    ANS[i][W+1]=0
    Q.append((i,0))
    Q.append((i,W+1))


for i in range(W+2):
    ANS[0][i]=0
    ANS[H+1][i]=0
    Q.append((0,i))
    Q.append((H+1,i))

for i in range(H):
    for j in range(W):
        if MAP[i][j]==".":
            Q.append((i+1,j+1))
            ANS[i+1][j+1]=0

while Q:
    x,y=Q.popleft()
    for z in [x-1,x,x+1]:
        for w in [y-1,y,y+1]:
            if 0<=z<H+2 and 0<=w<W+2 and ANS[z][w]>ANS[x][y]+1:
                ANS[z][w]=ANS[x][y]+1
                Q.append((z,w))

MAX=0
for ans in ANS:
    MAX=max(MAX,max(ans))
    #print(*ans)

print(MAX)