from collections import deque

H, W = map(int, input().split())

H += 2
W += 2

grid = "#" * W
for _ in range(H - 2):
    grid += "#" + input().split()
grid += "#" * W


def bfs(start):
    INF = 10 * 9
    dist = [INF] * (H * W)
    dist[start] = 0
    q = deque([start])
    while q:
        v = q.popleft()
        dv = dist[v]
        for move in (-1, 1, W, -W):
            # next place
            w = v + move
            # wall or not
            if grid[w] == "#":
                continue

            dw = dv + 1
            if dw >= dist[w]:
                continue
            dist[w] = dw
            q.append(w)

    return max(x for x in dist if x < INF)


answer = max(bfs(v) for v in range(H * W) if grid[v] != "#")
print(answer)
