from scipy.sparse.csgraph import dijkstra, csgraph_from_dense
import numpy as np

H, W = map(int, input().split())
S = [input() for _ in range(H)]

v_id_dict = {}
dista = np.ones((20, 20)) * np.inf

for i in range(H):
    for j in range(W):
        if S[i][j] == ".":
            v_id_dict[i, j] = len(v_id_dict)

            if (i > 0) and (S[i - 1][j] == "."):
                dista[v_id_dict[(i, j)], v_id_dict[(i - 1, j)]] = 1

            if (j > 0) and (S[i][j - 1] == "."):
                dista[v_id_dict[(i, j)], v_id_dict[(i, j - 1)]] = 1

dist_mat = dijkstra(
    csgraph_from_dense(dista[: len(v_id_dict), : len(v_id_dict)]),
    directed=False,
    unweighted=False,
)

print(int(dist_mat.max()))
