w,h = map(int,input().split())
b = [input() for _ in range(h)]

for i in range(h):
    for j in range(w):
        if b[i][j] == ".":
            si,sj = i,j

from collections import deque
INF = 999
dist = [[INF]*w for _ in range(h)]
dist[si][sj] = 0
q = deque([(si,sj)])

while q:
    vi,vj = q.popleft()
    for ni,nj in [(vi+1,vj),(vi,vj+1),(vi-1,vj),(vi,vj-1)]:
        if 0 <= ni < h and 0 < nj < w and dist[ni][nj] == INF:
            if b[ni][nj] == "#":
                dist[ni][nj] = dist[vi][vj] + 1
                q.append((ni,nj))
            else:
                dist[ni][nj] = dist[vi][vj]
                q.appendleft((ni,nj))
ans = 0
for i in range(h):
    for j in range(w):
        if b[i][j] == ".": ans = max(ans,dist[i][j])
print(ans)