#include <iostream>
#include <iomanip>
#include <string>
#include <stack>
#include <vector>
#include <math.h>
#include <stdio.h>
#include <algorithm>
#include <utility>
#include <functional>
#include <iterator>
#include <map>
#include <set>
#include <queue>
#include <list>
#include <regex>
#include <time.h>
#include <complex>
using namespace std;
using pii  = pair<int,int>;
using ll=long long;
using ld=long double;
#define pb push_back
#define mp make_pair
#define sc second
#define fr first
#define stpr setprecision
#define cYES cout<<"YES"<<endl
#define cNO cout<<"NO"<<endl
#define cYes cout<<"Yes"<<endl
#define cNo cout<<"No"<<endl
#define rep(i,n) for(ll i=0;i<(n);++i)
#define Rep(i,a,b) for(ll i=(a);i<(b);++i)
#define rrep(i,n) for(int i=n-1;i>=0;i--)
#define rRep(i,a,b) for(int i=a;i>=b;i--)
#define crep(i) for(char i='a';i<='z';++i)
#define psortsecond(A,N) sort(A,A+N,[](const pii &a, const pii &b){return a.second<b.second;});
#define ALL(x) (x).begin(),(x).end()
#define debug(v) cout<<#v<<":";for(auto x:v){cout<<x<<' ';}cout<<endl;
#define endl '\n'
int ctoi(const char c){
  if('0' <= c && c <= '9') return (c-'0');
  return -1;
}
ll gcd(ll a,ll b){return (b == 0 ? a : gcd(b, a%b));}
ll lcm(ll a,ll b){return a*b/gcd(a,b);}
constexpr ll MOD=1000000007;
constexpr ll INF=1000000011;
constexpr ll MOD2=998244353;
constexpr ll LINF = 1001002003004005006ll;
constexpr ld EPS=10e-8;
template <class T, class U> inline bool chmax(T& lhs, const U& rhs) { if (lhs < rhs) { lhs = rhs; return 1; } return 0; }
template <class T, class U> inline bool chmin(T& lhs, const U& rhs) { if (lhs > rhs) { lhs = rhs; return 1; } return 0; }
template<typename T> istream& operator>>(istream& is,vector<T>& v){for(auto&& x:v)is >> x;return is;}
template<typename T,typename U> istream& operator>>(istream& is, pair<T,U>& p){ is >> p.first; is >> p.second; return is;}
template<typename T,typename U> ostream& operator>>(ostream& os, const pair<T,U>& p){ os << p.first << ' ' << p.second; return os;}
template<class T> ostream& operator<<(ostream& os, vector<T>& v){
  for(auto i=begin(v); i != end(v); ++i){
    if(i !=begin(v)) os << ' ';
    os << *i;
  }
  return os;
}

ll A[1000][1000],B[10007],C,D,E,F,K,G,H,L,M,N,P,Q,R,W,X,Y,Z;
string S[100];
ll dp[1007][10007];
ll ma=0,mi=-1;

int main(){
  cin >> H >> W;
  rep(i,H){
    cin >> S[i];
  }
  rep(i,H){
    rep(j,W){
      rep(k,H){
        rep(l,W){
          A[k][l]=INF;
        }
      }
      queue<pair<ll,ll>> Q;
      ma=0;
      if(S[i][j]=='.'){
        Q.push(mp(i,j));
      }
      else{
        continue;
      }
      A[i][j]=0;
      while(!Q.empty()){
        auto X = Q.front();
        Q.pop();
        ll a=X.fr,b=X.sc;
        if(a!=0 && A[a-1][b]==INF && S[a-1][b]=='.'){
          A[a-1][b]=A[a][b]+1;
          Q.push(mp(a-1,b));
        }
        if(b!=0 && A[a][b-1]==INF && S[a][b-1]=='.'){
          A[a][b-1]=A[a][b]+1;
          Q.push(mp(a,b-1));
        }
        if(a!=H-1 && A[a+1][b]==INF && S[a+1][b]=='.'){
          A[a+1][b]=A[a][b]+1;
          Q.push(mp(a+1,b));
        }
        if(b!=W-1 && A[a][b+1]==INF && S[a][b+1]=='.'){
          A[a][b+1]=A[a][b]+1;
          Q.push(mp(a,b+1));
        }
      }
      rep(k,H){
        rep(l,W){
          if(S[k][l]=='.'){
            ma=max(A[k][l],ma);
          }
        }
      }
      mi=max(ma,mi);
    }
  }
  cout << mi << endl;
}

Measured memory_bytes: 2304