// Goods things are coming, just keep believing 
 
#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
 
using namespace __gnu_pbds;
using namespace std;
 
 
#define FOR(i, x, n) for(ll i = x; i < n; i++) 
#define pb push_back
#define pf push_front
#define ll unsigned long long
#define hii cout << "hii" << endl
#define okay cout << "okay" << endl
// #define hey cout << "hey" << endl
#define pii pair<int, int>
#define pll pair<ll, ll>
// #define mpp make_pair
#define endl '\n'
#define ff first 
#define int long long
#define ss second
#define vi vector<int>
#define all(s) s.begin(), s.end()
#define si size()
 #define ld long double
#define INF 1000000000;
 
template <class T> ostream& operator << (ostream &os, const vector<T> &v) { for (T i : v) os << i << ' '; return os; }
template <class T> ostream& operator << (ostream &os, const set<T> &v) { for (T i : v) os << i << ' '; return os; }
template <class T, class S> ostream& operator << (ostream &os, const pair<T, S> &v) { os << v.first << ' ' << v.second; return os; }
template <class T, class S> ostream& operator << (ostream &os, const unordered_map<T, S> &v) { for (auto i : v) os << '(' << i.first << "=>" << i.second << ')' << ' '; return os; } 
 
 
 
#ifndef ONLINE_JUDGE
#define trace(...) __f(#__VA_ARGS__, __VA_ARGS__)
    template <class Arg1> void __f(const char* name, Arg1&& arg1) { cerr << name << " : " << arg1 << endl; }
    template <class Arg1, class... Args>
    void __f(const char* names, Arg1&& arg1, Args&&... args) {
        const char* sep = strchr(names + 1, ',');
        cerr.write(names, sep - names) << " : " << arg1 << "  ";
        __f(sep + 1, args...);
    }
#else
#define trace(...) 0
#pragma GCC optimize ("O3")
#pragma GCC optimize ("unroll-loops")
#pragma GCC target("avx2,sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
#define _CRT_SECURE_NO_WARNINGS
#endif // ifndef ONLINE_JUDGE
 
typedef tree<
int,
null_type,
less<int>,
rb_tree_tag,
tree_order_statistics_node_update>
ordered_set;
 
 
 
const int N = 25;
const int LG = 20;
const int mod = 1e9 + 7;

char arr[N][N];
bool visited[N][N];

int n, m;
bool issafe(int x, int y)
{
	if(x < 0 or x >= n or y < 0 or y >= m or visited[x][y] or arr[x][y] == '#')
	{
		return 0;
	}
	return 1;
}

int f1(int i, int j, int k, int l)
{
	queue<pair<pair<int, int>, int>> q;
	if(issafe(i, j))q.push({{i, j}, 0});
	visited[i][j] = 1;
	while(q.size())
	{
		pair<int, int> curr = q.front().first;
		int dist = q.front().second;
		if(curr.first == k and curr.second == l)
		{
			return dist;
		}
		q.pop();
		if(issafe(curr.first + 1, curr.second))
		{
			q.push({{curr.first + 1, curr.second}, dist + 1});
			visited[curr.first + 1][curr.second] = 1;
		}
		if(issafe(curr.first - 1, curr.second))
		{
			q.push({{curr.first - 1, curr.second}, dist + 1});
			visited[curr.first - 1][curr.second] = 1;
		}
		if(issafe(curr.first, curr.second + 1))
		{
			q.push({{curr.first, curr.second + 1}, dist + 1});
			visited[curr.first][curr.second + 1] = 1;
		}
		if(issafe(curr.first, curr.second - 1))
		{
			q.push({{curr.first, curr.second - 1}, dist + 1});
			visited[curr.first][curr.second - 1] = 1;
		}
	}
	return -1;
}

void solve()
{
	cin >> n >> m;
	for(int i = 0; i < n; i++)
	{
		for(int j = 0; j < m; j++)
		{
			cin >> arr[i][j];

		}
	}
	int ret = 0;
	for(int i = 0; i < n; i++)
	{
		for(int j = 0; j < m; j++)
		{
			for(int k = 0; k < n; k++)
			{
				for(int l = 0; l < m; l++)
				{
					memset(visited, 0, sizeof(visited));
					int temp = f1(i, j, k, l);
					if(temp < 0) continue;
					ret = max(ret, temp);
				}
			}
		}
	}
	cout << ret << endl;
}
 
int32_t main()
{
	ios_base::sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL);
	int t = 1;
	// cin >> t;
	while(t--)
	{
		solve();
	}
}

Measured memory_bytes: 256