//  Created by conan1024hao in 2019.
//  Copyright © 2019 conan1024hao. All rights reserved.
//  専用ライブラリです、自由にコピーして構いません。
//  感谢看我的代码！Wechat:conan1024hao Twitter/QQ:810396815
#pragma GCC optimize ("O3")
#include <iostream>
#include <iomanip>
#include <istream>
#include <ostream>
#include <sstream>
#include <iterator>
#include <vector>
#include <algorithm>
#include <queue>
#include <deque>
#include <list>
#include <stack>
#include <map>
#include <unordered_map>
#include <set>
#include <utility>
#include <cmath>
#include <cstdio>
#include <cstring>
#include <string>
#include <ctime>
#include <cctype>
#include <cstdlib>
#define IINF 10e8
#define INF 10e16
#define MOD 1000000007
#define mod 1000000007
#define rep(i, a, n) for (ll i = a; i < (ll)(n); i++)
#define Endl endl
#define fi first
#define se second
#define pb push_back
#define mp make_pair
#define eb emplace_back
#define mmax(x,y)(x>y?x:y)
#define mmin(x,y)(x<y?x:y)
#define chmax(x,y) x=mmax(x,y)
#define chmin(x,y) x=mmin(x,y)
#define all(x) (x).begin(),(x).end()
#define siz(x) (ll)(x).size()
#define PI acos(-1.0)
using namespace std;
//using Int=int_fast64_t;
typedef long long int ll;
typedef pair<ll,ll>Pll;
typedef pair<int,int>Pin;
long long GCD(long long a, long long b) { return b ? GCD(b, a%b) : a; }
long long LCM(long long a, long long b)  {return a/GCD(a,b)*b;}
int dx[8]={-1,0,1,0,1,1,-1,-1};
int dy[8]={0,-1,0,1,1,-1,1,-1};
char dir[4]={'u','l','d','r'};
ll cmp1(pair<ll,string>a,pair<ll,string> b){
        if(a.fi!=b.fi)
        return a.fi<b.fi;
        else
        return a.se<b.se;
}

//---------------------------------------------------------------------------
long long modpow(long long a, long long n) {//a^n(MOD)を求める
    long long res = 1;
    while (n > 0) {
         if (n & 1) res = res * a % MOD;
         a= a * a % MOD;//次の準備
         n >>= 1;
      }
 return res;
}

bool color[60][60];int h,w;char s[200][100];int ans=0;
//---------------------------------------------------------------------------
int main(){//問題をちゃんと見ろ!!!!!!!!!!!!!!!!!      llか??????????        memset()!!!!!!!!!!!!       ペナを減らせ!!!!!!!!!!!!!
    cin.tie(0);
    ios::sync_with_stdio(false);
    //-------------------------------
    cin>>h>>w;int ans=0;
    for(int i=0;i<h;i++)
        for(int j=0;j<w;j++){
            cin>>s[i][j];
        }
    int d[60][60];
    for(int i=0;i<h;i++){
        for(int j=0;j<w;j++){
            if(s[i][j]=='#')continue;
            memset(color,0,sizeof(color));memset(d,0,sizeof(d));queue<Pin>Q;
             Q.push(mp(i,j));
            color[i][j]=1;
            while(!Q.empty()){
                   Pin p=Q.front();Q.pop();bool abc=0;
                   for(int i=0;i<4;i++){
                   int nx=p.fi+dx[i];
                   int ny=p.se+dy[i];
                       
                       if(nx>=0&&ny>=0&&nx<h&&ny<w){
                           if(s[nx][ny]=='#')
                               continue;
                           if(color[nx][ny]){
                               
                               continue;}
                           else{
                               color[nx][ny]=1;
                               d[nx][ny]=d[p.fi][p.se]+1;
                               Q.push(mp(nx,ny));abc=1;
                           }
                       }
                       
                   }
                
               }
         
            for(int i=0;i<h;i++){
            for(int j=0;j<w;j++){
                chmax(ans,d[i][j]);
            }}
        }
    }
    
    
    cout<<ans<<Endl;
    //-------------------------------
    return 0;
}
//---------------------------------------------------------------------------









Measured memory_bytes: 256