#include <bits/stdc++.h>
//#include <boost/multiprecision/cpp_int.hpp>
//#include <ext/pb_ds/assoc_container.hpp>
//#include <ext/pb_ds/tree_policy.hpp>
#pragma GCC optimize("O3")
#define REP(i,n) for(int i=0;i<n;i++)
#define REPP(i,n) for(int i=1;i<=n;i++)
#define ALL(obj) (obj).begin(), (obj).end()
#define EPS (1e-9)
#define INF (1e17)
#define PI (acos(-1))
//const double PI = acos(-1);
//const double EPS = 1e-15;
//long long INF=(long long)1E17;
#define i_7 (long long)(1e9+7)
//#define i_7 998'244'353
long mod(long a){
    long long c=a%i_7;
    if(c>=0)return c;
    return c+i_7;
}
long long po(long a, long b){
    if(b==0){
        return 1;
    }
    
    long long z = po(a,b/2);
    z = mod(z*z);
    if(b%2!=0){
        z = mod(a*z);
    }
    return z;
}

bool prime_(int n){
  if(n==1){
    return false;
  }else if(n==2){
    return true;
  }else{
    for(int i=2;i<=std::sqrt(n);i++){
      if(n%i==0){
        return false;
      }
    }
    return true;
  }
}

long long gcd_(long long a, long long b){
  if(a<b){
    std::swap(a,b);
  }
  if(a%b==0){
    return b;
  }else{
    return gcd_(b,a%b);
  }
}
 
long long lcm_(long long x, long long y){
  return (x/gcd_(x,y))*y;
}

//using namespace std;
//using namespace boost::multiprecision;
//using namespace __gnu_pbds;

int main(){
  using namespace std;
  int h, w;
  cin>>h>>w;
  vector<string> s(h);
  REP(i,h){
    cin>>s[i];
  }
  vector<vector<long long>> dist(h*w, vector<long long>(h*w, INF));
  REP(i, h*w){
    dist[i][i] = 0;
  }
  auto conv = [&](int i, int j){
    int res;
    res = w*i + j;
    return res;
  }
  int dx[] = {1, 0, -1, 0};
  int dy[] = {0, 1, 0, -1};
  REP(i,h){
    REP(j,w){
      if(s[i][j] == '#')continue;
      REP(k,4){
        int ni = i + dx[k];
        int nj = j + dy[k];
        if(ni < 0 || ni >= h || nj < 0 || nj >= w){
          continue;
        }
        if(s[ni][nj] == '#')continue;
        int f = conv(i, j);
        int t = conv(ni, nj);
        dist[f][t] = dist[t][f] = 1;
      }
    }
  }
  REP(k,h*w){
    REP(i,h*w){
      REP(j,h*w){
        dist[i][j] = min(dist[i][j], dist[i][k] + dist[k][j]);
      }
    }
  }
  long long ans = 0;
  REP(i,h*w){
    REP(j,h*w){
      if(dist[i][j] == INF)continue;
      ans = max(ans, dist[i][j]);
    }
  }
  cout<<ans<<endl;
  return 0;
}
