Problem:
Score : 400 points

Problem Statement

Takahashi has a maze, which is a grid of H \times W squares with H horizontal rows and W vertical columns.

The square at the i-th row from the top and the j-th column is a "wall" square if S_{ij} is #, and a "road" square if S_{ij} is ..

From a road square, you can move to a horizontally or vertically adjacent road square.

You cannot move out of the maze, move to a wall square, or move diagonally.

Takahashi will choose a starting square and a goal square, which can be any road squares, and give the maze to Aoki.

Aoki will then travel from the starting square to the goal square, in the minimum number of moves required.

In this situation, find the maximum possible number of moves Aoki has to make.

Constraints

1 \leq H,W \leq 20

S_{ij} is . or #.

S contains at least two occurrences of ..

Any road square can be reached from any road square in zero or more moves.

Input

Input is given from Standard Input in the following format:

H W
S_{11}...S_{1W}
:
S_{H1}...S_{HW}

Output

Print the maximum possible number of moves Aoki has to make.

Sample Input 1

3 3
...
...
...

Sample Output 1

4

If Takahashi chooses the top-left square as the starting square and the bottom-right square as the goal square, Aoki has to make four moves.

Sample Input 2

3 5
...#.
.#.#.
.#...

Sample Output 2

10

If Takahashi chooses the bottom-left square as the starting square and the top-right square as the goal square, Aoki has to make ten moves.

Solution in C++14 (GCC 5.4.1) (judge verdict: Accepted)

/*
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...````` `  ` 7a,.dM{....`...../MMMN......(J=`  .>......._(/=             `  ` `````...~~~:
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~~j,..```````.D??>>>MM/....`........(-=`                          `  ` ` ```````...~~~:::;>
~~~j,...````.E??>??>?MN-.........(J=                            `   ` ` ``````....~~~~::;??
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;:::~~/e.(K==?????????<dM"!                          `    `   ` ` `` ``````...~~~~:::;>??=l
@TT_beginner
*/
#include "bits/stdc++.h"
using namespace std;
#define ok1 printf("ok1\n")
#define ok2 printf("ok2\n")
#define M LLONG_MAX
#define rep(i,n) for(int i=0;i<n;++i)
#define REP(i,s,n) for(int i=(s);i<(n);++i)
#define repr(i,n) for(int i=n-1;i>=0;--i)
#define REPR(i,s,n) for(int i=(s);i>=(n);--(i))
#define all(a) (a).begin(),(a).end()
#define reall(a) (a).rbegin(),(a).rend()
#define pb emplace_back //emplace_backの方が速いが使い慣れてないため
#define DOUBLE fixed << setprecision(15)
#define fi first
#define se second
#define mp make_pair
#define mt make_tuple
const double pi = acos(-1.0L);
typedef vector<int> vi;
typedef vector<string> vs;
typedef long long ll;
typedef vector<ll> vll;
typedef vector<vi> vvi;
typedef vector<vll> vvll;
typedef vector<char> vc;
typedef vector<double> vd;
typedef vector<bool> vb;
typedef deque<ll> dll;
typedef pair<ll, ll> P;
typedef tuple<ll, ll, ll> TLL;
typedef vector<P> vP;
typedef vector<TLL> vTLL;
const ll mod = 1e9 + 7;
//const ll mod = 998244353;
ll dy[4] = { 1,0,-1,0 };
ll dx[4] = { 0,1,0,-1 };
template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return true; } return false; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return true; } return false; }
template<class A, class B>
ostream& operator<<(ostream& ost, const pair<A, B>&p) {
	ost << "{" << p.first << ", " << p.second << "} ";
	return ost;
}
template<class T>
ostream& operator<<(ostream& ost, const vector<T>&v) {
	ost << "{";
	for (int i = 0; i<v.size(); i++) {
		if (i)ost << ", ";
		ost << v[i];
	}
	ost << "} ";
	return ost;
}
template<class A, class B>
ostream& operator<<(ostream& ost, const map<A, B>&v) {
	ost << "{";
	for (auto p : v) {
		ost << "{" << p.first << ", " << p.second << "} ";
	}
	ost << "} ";
	return ost;
}
bool out_check(ll a, ll b) { return (0 <= a && a < b); }
double pitagoras(ll a, ll b, ll c, ll d) {
	double dx = a - b, dy = c - d;
	return pow(dx * dx + dy * dy, 0.5);
}
ll modpow(ll a, ll b) { ll c = 1; while (b > 0) { if (b & 1) { c = a * c%mod; }a = a * a%mod; b >>= 1; }return c; }
void Yes(bool x) { cout << ((x) ? "Yes\n" : "No\n"); }
void YES(bool x) { cout << ((x) ? "YES\n" : "NO\n"); }
void yes(bool x) { cout << ((x) ? "yes\n" : "no\n"); }
void Yay(bool x) { cout << ((x) ? "Yay!\n" : ":(\n"); }
vector<string> DAY = { "SUN", "MON", "TUE", "WED", "THU", "FRI", "SAT" };
ll  n, m, d, e, r, l, k, h, Q, ans, ret = M;
bool flag = false, flag2 = false, flag3 = false;
//string s, t, u;

//最大公約数
ll gcd(ll a, ll b) {
	if (a < b)swap(a, b);
	while (b) {
		ll r = a % b; a = b; b = r;
	}
	return a;
}
//最小公倍数
ll lcm(ll a, ll b) {
	return (a * b / gcd(a, b));
}
int main() {
	cin.tie(0); ios::sync_with_stdio(false);//not intractive
	ll w;
	cin >> h >> w;
	vs s(h);
	rep(i, h) {
		cin >> s[i];
		rep(j, w) if (s[i][j] == '#')flag = true;
	}
	if (!flag) {
		cout << h + w - 2 << endl;
		return 0;
	}
	rep(i, h) {
		rep(j, w) {
			if (s[i][j] != '#') {
				queue<pair<P, ll>> q;
				q.push(mp(P(i, j), 0));
				vvll v(h, vll(w));
				v[i][j] = 1;
				ll cnt = 0;
				while (!q.empty()) {
					pair<P, ll> x = q.front(); q.pop();
					P p = x.first;
					ll cost = x.second;
					chmax(cnt, cost);
					rep(k, 4) {
						//cout << p << " " << dx[k] << " " << dy[k] << endl;
						if (p.first + dx[k] >= 0 && p.first + dx[k] < h && p.second + dy[k] >= 0 && p.second + dy[k] < w && !v[p.first + dx[k]][p.second + dy[k]] && s[p.first + dx[k]][p.second + dy[k]] != '#') {
							q.push(mp(P(p.first + dx[k], p.second + dy[k]), cost + 1));
							v[p.first + dx[k]][p.second + dy[k]] = 1;
						}
					}
				}
				chmax(ans, cnt);
				//cout << "ans is: " << ans << endl;
			}
		}
	}
	cout << ans << endl;
}