# -*- coding: utf-8 -*-
import sys
import math
import os
import itertools
import string
import heapq
import _collections
from collections import Counter
from collections import defaultdict
from functools import lru_cache
import bisect
import re
import queue


class Scanner():
    @staticmethod
    def int():
        return int(sys.stdin.readline().rstrip())

    @staticmethod
    def string():
        return sys.stdin.readline().rstrip()

    @staticmethod
    def map_int():
        return [int(x) for x in Scanner.string().split()]

    @staticmethod
    def string_list(n):
        return [input() for i in range(n)]

    @staticmethod
    def int_list_list(n):
        return [Scanner.map_int() for i in range(n)]

    @staticmethod
    def int_cols_list(n):
        return [int(input()) for i in range(n)]


class Math():
    @staticmethod
    def gcd(a, b):
        if b == 0:
            return a
        return Math.gcd(b, a % b)

    @staticmethod
    def lcm(a, b):
        return (a * b) // Math.gcd(a, b)

    @staticmethod
    def roundUp(a, b):
        return -(-a // b)

    @staticmethod
    def toUpperMultiple(a, x):
        return Math.roundUp(a, x) * x

    @staticmethod
    def toLowerMultiple(a, x):
        return (a // x) * x

    @staticmethod
    def nearPow2(n):
        if n <= 0:
            return 0
        if n & (n - 1) == 0:
            return n
        ret = 1
        while(n > 0):
            ret <<= 1
            n >>= 1
        return ret

    @staticmethod
    def sign(n):
        if n == 0:
            return 0
        if n < 0:
            return -1
        return 1

    @staticmethod
    def isPrime(n):
        if n < 2:
            return False
        if n == 2:
            return True
        if n % 2 == 0:
            return False
        d = int(n ** 0.5) + 1
        for i in range(3, d + 1, 2):
            if n % i == 0:
                return False
        return True


class PriorityQueue:
    def __init__(self, l=[]):
        self.__q = l
        heapq.heapify(self.__q)
        return

    def push(self, n):
        heapq.heappush(self.__q, n)
        return

    def pop(self):
        return heapq.heappop(self.__q)


MOD = 998244353
INF = int(1e15)


def bfs(S, H, W, s):
    q = []
    q.append(s)
    M = [[-1 for _ in range(W)]for _ in range(H)]
    M[s[1]][s[0]] = 0
    while q != []:
        p = q.pop(0)
        DIR = [(-1, 0), (1, 0), (0, 1), (0, -1)]
        for i in range(4):
            n = (p[0] + DIR[i][0], p[1] + DIR[i][1])
            if not (0 <= n[0] < W and 0 <= n[1] < H):
                continue
            if S[n[1]][n[0]] == "#":
                continue
            if M[n[1]][n[0]] != -1:
                continue
            M[n[1]][n[0]] = M[p[1]][p[0]] + 1
            q.append(n)
    res = 0
    for h in range(H):
        res = max(res, max(M[h]))
    return res


def main():
    # sys.stdin = open("sample.txt")
    H, W = Scanner.map_int()
    S = Scanner.string_list(H)
    ans = 0
    for h in range(H):
        for w in range(W):
            ans = max(ans, bfs(S, H, W, (w, h)))
    print(ans)
    return


if __name__ == "__main__":
    main()
