#include<bits/stdc++.h>
using namespace std;
using ll = long long;

double per(double n){
    if(n == 1) return n;
    return n*per(n-1);
}

int main(){
    int n;
    cin >> n;
    vector<pair<int, int>>p(n);
    for(int i=0;i<n;i++){
        int x, y;
        cin >> x >> y;
        p[i] = {x, y};
    }
    double ans = 0;

    sort(p.begin(), p.end());

    do{
        for(int i=0;i<n;i++){
            if(i){
                ans += sqrt((p[i].first-p[i-1].first)*(p[i].first-p[i-1].first)+(p[i].second-p[i-1].second)*(p[i].second-p[i-1].second));
            }
        }
    }while(next_permutation(p.begin(), p.end()));

    ans /= per((double)n);
    printf("%.9lf\n", ans);

}
#include"bits/stdc++.h"
using namespace std;
#define ll long long
#define mod 1000000007
#define prec(n) fixed<<setprecision(n)
int N;
int x[10], y[10];
 
double dist(int i, int j) {
    double dx = x[i] - x[j];
    double dy = y[i] - y[j];
    return pow(dx * dx + dy * dy, 0.5);
}
 
void solve() {
    cin >> N;
    for(int i = 1; i <= N; i++) cin >> x[i] >> y[i];

    double dsum = 0.0;
	
	for (int i = 1 ;i<=N; ++i) {
		for (int j = i+1; j<=N;++j) {
			dsum += dist(i, j) * 2 / N;
		}
	}

	cout << prec(10) << dsum << "\n";

    return;
}
 
int main() {
    solve();
    return 0;
}