h, w = map(int, input().split())
grid = []
for _ in range(h):
    grid.append(input())
res = 0
for i in range(h):
    for j in range(w):
        if grid[i][j] != '.':
            continue
        visited = []
        queue = [(i, j, 0)]
        while queue:
            c_i, c_j, level = queue.pop(0)
            visited.append((c_i, c_j))
            for n_i, n_j in ((c_i - 1, c_j), (c_i + 1, c_j), (c_i, c_j - 1), (c_i, c_j + 1)):
                if 0 <= n_i < h and 0 <= n_j < w and grid[n_i][n_j] == '.' and (n_i, n_j) not in visited:
                    queue.append((n_i, n_j, level + 1))
        res = max(res, level)
print(res)        