import sys
from collections import deque

sr = lambda: sys.stdin.readline().rstrip()
ir = lambda: int(sr())
lr = lambda: list(map(int, sr().split()))

def bfs(sy, sx):
    dist = [[-INF] * W for _ in range(H)]
    dist[sy][sx] = 0
    que = deque([(sy, sx)])
    popleft = que.popleft; append = que.append
    while que:
        y, x = popleft()
        cur = dist[y][x]
        for dy, dx in zip([0, 1, 0, -1], [1, 0, -1, 0]):            
            ny = y + dy
            nx = x + dx
            if 0 <= ny < H and 0 <= nx < W and \
                    grid[ny][nx] != '#' and dist[ny][nx] == -INF:
                dist[ny][nx] = cur + 1
                append((ny, nx))
    ret = max(max(x for x in di)for di in dist)
    return ret

INF = 10 ** 3
H, W = lr()
grid = [sr() for _ in range(H)]
answer = max(max(bfs(i, j) for j in range(W)) for i in range(H))
print(answer)
# 22