from collections import deque

def bfs(grid, start):
    H, W = len(maze), len(maze[0])
    distance = [[-1] * W for _ in range(H)]
    queue = deque([start])
    distance[start[0]][start[1]] = 0
    while queue:
        x, y = queue.popleft()
 
        for i, j in ((1, 0), (-1, 0), (0, 1), (0, -1)):
            new_y, new_x = y + j, x + i
            if 0 <= new_y < W and 0 <= new_x < H:
                if grid[new_x][new_y] == '.' and distance[new_x][new_y] == -1:
                    distance[new_x][new_y] = distance[x][y] + 1
                    queue.append((new_x, new_y))
    return max(map(max, distance))

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


res = -1
for i in range(h):
    for j in range(w):
        if(maze[i][j] == '#'): continue
        tmpmaze = copy.deepcopy(maze)
        steps = bfs(tmpmaze, [i, j])
        if(steps > res):
            res = steps
print(res)