#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ull = unsigned long long;
 
#define REP(i,n) for(long long i = 0; i < (n); i++)
#define FOR(i, m, n) for(long long i = (m);i < (n); ++i)
#define ALL(obj) (obj).begin(),(obj).end()
#define SPEED cin.tie(0);ios::sync_with_stdio(false);
 
template<class T> using V = vector<T>;
template<class T, class U> using P = pair<T, U>;
template<class T> using PQ = priority_queue<T>;
template<class T> using PQR = priority_queue<T,vector<T>,greater<T>>;
 
constexpr long long MOD = (long long)1e9 + 7;
constexpr long long MOD2 = 998244353;
constexpr long long HIGHINF = (long long)1e18;
constexpr long long LOWINF = (long long)1e15;
constexpr long double PI = 3.1415926535897932384626433;
 
template <class T> vector<T> multivector(size_t N,T init){return vector<T>(N,init);}
template <class... T> auto multivector(size_t N,T... t){return vector<decltype(multivector(t...))>(N,multivector(t...));}
template <class T> void corner(bool flg, T hoge) {if (flg) {cout << hoge << endl; exit(0);}}
template <class T, class U>ostream &operator<<(ostream &o, const map<T, U>&obj) {o << "{"; for (auto &x : obj) o << " {" << x.first << " : " << x.second << "}" << ","; o << " }"; return o;}
template <class T>ostream &operator<<(ostream &o, const set<T>&obj) {o << "{"; for (auto itr = obj.begin(); itr != obj.end(); ++itr) o << (itr != obj.begin() ? ", " : "") << *itr; o << "}"; return o;}
template <class T>ostream &operator<<(ostream &o, const multiset<T>&obj) {o << "{"; for (auto itr = obj.begin(); itr != obj.end(); ++itr) o << (itr != obj.begin() ? ", " : "") << *itr; o << "}"; return o;}
template <class T>ostream &operator<<(ostream &o, const vector<T>&obj) {o << "{"; for (int i = 0; i < (int)obj.size(); ++i)o << (i > 0 ? ", " : "") << obj[i]; o << "}"; return o;}
template <class T, class U>ostream &operator<<(ostream &o, const pair<T, U>&obj) {o << "{" << obj.first << ", " << obj.second << "}"; return o;}
template <template <class tmp>  class T, class U> ostream &operator<<(ostream &o, const T<U> &obj) {o << "{"; for (auto itr = obj.begin(); itr != obj.end(); ++itr)o << (itr != obj.begin() ? ", " : "") << *itr; o << "}"; return o;}
void print(void) {cout << endl;}
template <class Head> void print(Head&& head) {cout << head;print();}
template <class Head, class... Tail> void print(Head&& head, Tail&&... tail) {cout << head << " ";print(forward<Tail>(tail)...);}
template <class T> void chmax(T& a, const T b){a=max(a,b);}
template <class T> void chmin(T& a, const T b){a=min(a,b);}
void YN(bool flg) {cout << (flg ? "YES" : "NO") << endl;}
void Yn(bool flg) {cout << (flg ? "Yes" : "No") << endl;}
void yn(bool flg) {cout << (flg ? "yes" : "no") << endl;}

#include <bits/stdc++.h>
using namespace std;

int main() {
	int H,W; cin >> H >> W;
	vector<vector<int>> edge(H*W,vector<int>(H*W,1234567));
	vector<string> S(H);
	for(int i = 0; i < H; ++i){
		cin >> S[i];
	}
	for(int i = 0; i < H; ++i){
		for(int j = 0; j < W; ++j){
			if(S[i][j]=='#') continue;
			int s,t;
			s=i+1,t=j;
			if(0<=s&&s<H&&0<=t&&t<W&&S[s][t]=='.'){
				edge[i*W+j][s*W+t]=1;
			}
			s=i-1,t=j;
			if(0<=s&&s<H&&0<=t&&t<W&&S[s][t]=='.'){
				edge[i*W+j][s*W+t]=1;
			}
			s=i,t=j+1;
			if(0<=s&&s<H&&0<=t&&t<W&&S[s][t]=='.'){
				edge[i*W+j][s*W+t]=1;
			}
			s=i,t=j-1;
			if(0<=s&&s<H&&0<=t&&t<W&&S[s][t]=='.'){
				edge[i*W+j][s*W+t]=1;
			}
		}
	}
	for(int k = 0; k < H*W; ++k) for(int i = 0; i < H*W; ++i) for(int j = 0; j < H*W; ++j) chmin(edge[i][j],edge[i][k]+edge[k][j]);
	int ans = 0;
	for(int i = 0; i < H*W; ++i) for(int j = 0; j < H*W; ++j) if(i!=j&&edge[i][j]!=1234567) chmax(ans,edge[i][j]);
    cout << ans << endl;
	return 0;
}

Measured memory_bytes: 896