%%time

import numpy as np
h, w = list(map(int, input().split()))
ss = []
for _ in range(h):
    s = input()
#     s = s.replace(".", "0").replace("#", "1")
#     s = list(map(int, list(s)))
    ss.append(s)
# ss = np.array(ss)
    

from collections import deque
def find(now_h, now_w):
    dp = np.zeros([h, w], dtype = np.int32) + 9999
    dp[now_h, now_w] = 0
    que = deque()
    que.append((now_h, now_w))
    while len(que) > 0:
        now_h, now_w = que.pop()
        if (now_h, now_w) in que:
            continue
        for mh, mw in ([0,1], [0,-1], [1,0], [-1,0]):
            next_h = now_h + mh
            next_w = now_w + mw
            if next_h < 0 or next_h >= h or next_w < 0 or next_w >= w:
                continue
            if ss[next_h][next_w] == "#":
                continue
            a = dp[now_h, now_w] + 1
            if dp[next_h, next_w] > a:
                dp[next_h, next_w] = a
                que.append((next_h, next_w))
    score = np.max(dp[dp != 9999])
    return score

maximum = 0
for sh in range(h):
    for sw in range(w):
        if ss[sh][sw] == ".":
            score = find(sh, sw)
            if maximum < score:
                maximum = score

print(maximum)