import numpy as np
from scipy.sparse import csr_matrix, csgraph
H, W = map(int, input().split())
maze = "".join(input() for _ in range(H))
array = np.zeros((H * W, H * W), dtype=np.bool)
for n in range(H * W):
    if maze[n] == "#":
        continue
    r, c = divmod(n, W)
    if c and maze[n - 1] == ".":
        array[n][n - 1] = 1
    if c != W - 1 and maze[n + 1] == ".":
        array[n][n + 1] = 1
    if r and maze[n - W] == ".":
        array[n][n - W] = 1
    if r != H - 1 and maze[n + W] == ".":
        array[n][n + W] = 1
dist = csgraph.shortest_path(csr_matrix(array, dtype=np.bool, shape=(H * W, H * W)), directed=False)
print(int(dist[dist < 401].max()))