import copy, collections, itertools
H, W = [int(_) for _ in input().split()]
S_ = [['#'] * (W + 2)] + [['#'] + list(input()) + ['#']
                          for _ in range(H)] + [['#'] * (W + 2)]
ans = 0
for i, j in itertools.product(range(1, H + 1), range(1, W + 1)):
    if S_[i][j] == '.':
        S = copy.deepcopy(S_)
        S[i][j] = '#'
        q = collections.deque([(i, j, 0)])
        while q:
            x, y, d = q.popleft()
            for dx, dy in [[-1, 0], [1, 0], [0, -1], [0, 1]]:
                if S[x + dx][y + dy] == '.':
                    X, Y = x + dx, y + dy
                    S[X][Y] = '#'
                    q += [(X, Y, d + 1)]
        ans = max(d, ans)
print(ans)
