INF = 1e18
from collections import deque
W, H = map(int, input().split())
C = [input() for _ in range(H)]

# start地点探す
for i in range(H):
    for j in range(W):
        if C[i][j] == '.':
            si, sj = i, j
            break
    else:
        continue
    break

dist = [[INF] * (W) for _ in range(H)]
dist[si][sj] = 0
que = deque()
que.append((si, sj))

def check(i, j):
    return 0 <= i < H and 0 <= j < W

while que:
    i, j = que.popleft()
    for ii, jj in [(0, -1), (0, 1), (-1, 0), (1, 0)]:
        ni, nj = i+ii, j+jj
        if not check(ni, nj):
            continue
        if C[ni][nj] == '.':
            x = dist[i][j]
            # 短くできるなら
            if dist[ni][nj] > x:
                dist[ni][nj] = x
                que.appendleft((ni, nj))
        else:
            x = dist[i][j] + 1
            if dist[ni][nj] > x:
                dist[ni][nj] = x
                que.append((ni, nj))

ans = 0
for i in range(H):
    for j in range(W):
        if C[i][j] == '.':
            ans = max(ans, dist[i][j])
print(ans)