h,w = map(int,input().split())
b = ["."*(w+2)]
for _ in range(h):
    b.append("."+input()+".")
b += ["."*(w+2)]
h += 2; w += 2
dist = [[-1]*w for _ in range(h)]
from collections import deque
q = deque()
for i in range(h):
    for j in range(w):
        if b[i][j] == ".":
            q.append((i,j))
            dist[i][j] = 0
while q:
    i,j = q.popleft()
    for di in range(-1,2):
        for dj in range(-1,2):
            ni = i+di
            nj = j+dj
            if 0 <= ni < h and 0 <= nj < w and dist[ni][nj] == -1:
                dist[ni][nj] = dist[i][j] + 1
                q.append((ni,nj))
print(dist[i][j])