long long A,B,C,D,E,F,    I,J,K,  M,N,O,P,Q,R,S,T,U,V,  X,Y,Z;
//long long A,B,C,D,E,F,  H,I,J,K,L,M,N,O,P,Q,R,S,T,U,V,W,X,Y,Z;
double dA,dB,dC,dD,dE,dF,   dI,dJ,dK,  dM,dN,dO,dP,dQ,dR,dS,dT,dU,dV,  dX,dY,dZ;
#include <string>
#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#include <iostream>
#include <algorithm>
#include <vector>
#include <queue>
#include <string>
#include <iomanip>
#include <cmath>

//typedef pair<long long,long long> P;
//typedef pair<long long,P> P1;
//typedef pair<P,P> P2;
#define pu push
#define pb push_back
#define mp make_pair
#define eps 1e-7
//#define INF 100000000000000000//19桁
#define INF 1e18+5//19桁
#define mod 1000000007
#define fi first
#define se second
#define rep(i,x) for(long long i=0;i<x;i++)
#define repn(i,x) for(long long i=1;i<=x;i++)
#define REP(i,n,x) for(long long i=n;i<x;i++)
#define REPN(i,n,x) for(long long i=n+1;i<x;i++)
#define repn(i,x) for(long long i=1;i<=x;i++)
#define rrep(i,x) for(long long i=x-1;i>=0;i--)
#define rrepn(i,x) for(long long i=x;i>1;i--)
#define SORT(x) sort(x.begin(),x.end())
#define ERASE(x) x.erase(unique(x.begin(),x.end()),x.end())
#define POSL(x,v) (lower_bound(x.begin(),x.end(),v)-x.begin())
#define POSU(x,v) (upper_bound(x.begin(),x.end(),v)-x.begin())
#define All(x) (x).begin(),(x).end()
#define rAll(x) (x).rbegin(),(x).rend()

#define ALL(v)  (v).begin(),(v).end()
//#define MAX(a,b) if(a>b)
#define MAX(a,b) (((a)>(b))?(a):(b))
#define MIN(a,b) (((a)<(b))?(a):(b))
#define popcount __builtin_popcount

const string MINUSINF = "-";

template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return 1; } return 0; }
void chmax(string &a, string b) {
    if (a == MINUSINF) a = b;
    else if (a.size() < b.size()) a = b;
    else if (a.size() == b.size()) {
        if (a < b) a = b;
    }
}


template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return 1; } return 0; }

#define pr printf
#define re return
#define CASET int ___T, case_n = 1; scanf("%d ", &___T); while (___T-- > 0)


int GCD(int a, int b) { return b ? GCD(b, a%b) : a; }

stack<long long> sta;   
queue<long long> que;
deque<long long> dque;

set <string> s_set;








long long sum,sum1,sum2,sum3,sum4;
long long flg,flg1,flg2,flg3,cnt,cnt1,cnt2,cnt3,cnt4;
long long wk,wk1,wk2,wk3,wk4;
long long ans;

double dsum,dsum1,dsum2,dsum3,dsum4;
double dcnt,dcnt1,dcnt2,dcnt3,dcnt4;
double dwk,dwk1,dwk2,dwk3,dwk4;
double dans;
double dmax,dmax1;
double dmin,dmin1;



bool f;

long long weight[110], value[110]; // 品物の個数は 100 個なので少し余裕持たせてサイズ 110 に
ll a[100010][3]; // a[i], b[i], c[i] をそれぞれまとめて a[i][0], a[i][1], a[i][2] にしてしまう
long long n,h[100005];
vector<long long> p, c;
int vis[200001];

vector<string> fi;
int Left[2100][2100], Right[2100][2100], Up[2100][2100], Down[2100][2100];

long long match[10] = {2,5,5,4,5,6,3,7,6};
vector<long long> box;


//	BFS		(AGC033 A  Darker and Darker)
#define P pair<int,int>
using namespace std;

// マスの縦横]
int H,W;
// #,.を格納するマス
char MAT[1000][1000];
char MAT2[1000][1000];

// 幅優先探索の深さを格納するマス
int depth[1000][1000];
// 隣り合う辺への移動量
int dx[4] = {0, 1, 0, -1};
int dy[4] = {1, 0, -1, 0};
// 答え
	
void BFS(queue<P> que){
	

	// 入力を受取り、キューに位置x,yを格納
	
	while (!que.empty()){
		P p = que.front();
		que.pop();
		for (int i=0;i<4;i++){
			int nx = p.first + dx[i];
			int ny = p.second + dy[i];
			if (nx>=0 && nx<H && ny>=0 && ny<W && MAT2[nx][ny]!='#'){
				MAT2[nx][ny] = '#';
				depth[nx][ny] = depth[p.first][p.second]+1;
				que.push(P(nx,ny));
			}
		}
	}
}


ll depth2;
int main(){


	queue<P> que;
	cin >> H >> W;
	for (int i=0;i<H;i++){
		for (int j=0;j<W;j++){
			cin >> MAT[i][j];
			MAT2[i][j]= MAT[i][j];
		}
	}
	for (int i=0;i<H;i++){
		for (int j=0;j<W;j++){
			if (MAT[i][j]=='.'){
				MAT2[i][j]='#';
				for (int i=0;i<H;i++){
					for (int j=0;j<W;j++){
						depth[i][j]=0;
					}
				}
				depth2=0;
				std::queue<P> empty;
   				std::swap( que, empty );
				que.push(P(i,j));
				BFS(que);
				for (int i=0;i<H;i++){
					for (int j=0;j<W;j++){
						depth2=MAX(depth[i][j],depth2);
					}
				}
				

				ans=MAX(ans,depth2);
				for (int i=0;i<H;i++){
					for (int j=0;j<W;j++){
						MAT2[i][j]= MAT[i][j];
					}
				}
			}
		}
	}
	
	
	cout << ans << endl;
	return 0;
}

Measured memory_bytes: 6528