#include <bits/stdc++.h>          
using namespace std;
typedef long long ll;
typedef long double ld;
/*
void printVector(const vector<int>& vec) {
    for (int value : vec) {
        cout << value << " ";
    }
    cout << endl;
}
*/
int main() {
    int N;
    cin >> N;
    vector<ld> x(N), y(N);
    for (int i=0; i<N; i++) cin >> x[i] >> y[i];
    vector<ld> base(N), L(N-1);
    for (int i=0; i<N; i++) base[i] = i;
    int p=1;
    for (int i=1; i<=N; i++) p*=i;
    ld cnt = 0;
    for (int i=0; i<p; i++) {
        for (int j=0; j<N-1; j++) {
            cnt += sqrt(pow(x[base[j+1]]-x[base[j]],2)+pow(y[base[j+1]]-y[base[j]],2));
        }
        next_permutation(base.begin(),base.end());
    }
    ld ans = cnt/p;
    cout << fixed << setprecision(8);
    cout << ans << endl;
}
#include <bits/stdc++.h>
#define rep(i,n) for(int i=0;i<(n);i++)
#define all(v) v.begin(),v.end()
#define len(x) (ll)(x).length()
using namespace std;
typedef long long ll;
typedef pair<int,int> P;
const int INF=1e9;
const int di[] = {-1,0,1,0};
const int dj[] = {0,-1,0,1};

int main(){
  double n;
  cin>>n;
  vector<vector<double>> d(n,vector<double>(2));
  rep(i,n)rep(j,2) cin>>d[i][j];
  vector<int> t(n);
  rep(i,n) t[i]=i;
  double sum=0;
  int cnt=0;
  do{
    for(int i=1;i<n;i++){
      sum+=sqrt((d[t[i]][0]-d[t[i-1]][0])*(d[t[i]][0]-d[t[i-1]][0])+(d[t[i]][1]-d[t[i-1]][1])*(d[t[i]][1]-d[t[i-1]][1]));
    }
    cnt++;
  }while(next_permutation(all(t)));
  printf("%.10f\n",sum/cnt);
}