/**
 * code generated by JHelper
 * More info: https://github.com/AlexeyDmitriev/JHelper
 * @author champon
 */

#include <iostream>
#include <fstream>
#include<bits/stdc++.h>
#define se cout << "test ok" << endl;
#define sp << " " <<
#define enl "\n"
using namespace std;
using ll=long long;
using ld=long double;
ll mod = ll(1e9)+7;
ll inf = ll(1e18);
void bl(bool b, ostream& out=cout){ out << (b ? "Yes" : "No") << endl; }
void bl2(bool b, ostream& out=cout){ out << (b ? "YES" : "NO") << endl; }
template<typename T> inline bool chmax(T &a, T b){ if(a<b){a=b; return true;} return false; }
template<typename T> inline bool chmin(T &a, T b){ if(a>b){a=b; return true;} return false; }
int dx[] = {1, 0, -1, 0, 1, -1, 1, -1};
int dy[] = {0, 1, 0, -1, 1, -1, -1, 1};

#ifndef INCLUDE_DEBUG_HPP
#define INCLUDE_DEBUG_HPP

#include <iostream>
#include <vector>
#include <map>
using namespace std;

namespace dbg {
    using ll = long long;

    class Debug {
    public:
        Debug(){}
        template<typename T>
        void vd(vector<T> vec);
        template<typename T>
        void vd2(vector<vector<T>> vec);
        template<typename T>
        void ad(T *arr, ll n);
        template<typename T, size_t N>
        void ad2(T (&arr)[N], ll n);
        template<typename T>
        void md(T mp);
    };

    template<typename T>
    void Debug::vd(vector<T> vec) {
        for (auto vv : vec) cout << vv << " ";
        cout << endl;
    }

    template<typename T>
    void Debug::vd2(vector<vector<T>> vec) {
        for (auto vv : vec) {
            for (auto vvv : vv) cout << vvv << " ";
            cout << endl;
        }
    }

    template<typename T>
    void Debug::ad(T *arr, ll n) {
        for (int i = 0; i <= n; i++) cout << arr[i] << " ";
        cout << endl;
    }

    template<typename T, size_t N>
    void Debug::ad2(T (&arr)[N], ll n) {
        for (int i = 0; i <= N; i++) for (int j = 0; j <= n; j++) cout << arr[i][j] << " ";
        cout << endl;
    }

    template<typename T>
    void Debug::md(T mp) {
        cout << "key : value" << endl;
        for (auto const &m : mp) cout << m.first << " : " << m.second << endl;
        cout << endl;
    }

}

#endif //INCLUDE_DEBUG_HPP

using dbg::Debug;
Debug debug;

class CAverageLength {
public:
    struct Point{
        int x, y;
        Point(int x, int y):x(x),y(y){}
        bool operator<(Point const& rhs) {
            return x < rhs.x;
        }
    };
    double distance(Point p1, Point p2){
        return sqrt(pow(p1.x-p2.x, 2) + pow(p1.y-p2.y, 2));
    }
	void solve(std::istream& in, std::ostream& out) {
	    int n;
	    in >> n;
        vector<Point> p;
        int x, y;
        for(int i=0; i<n; i++){
            in >> x >> y;
            p.emplace_back(x, y);
        }

        double sum = 0.0;
        vector<int> v(n);
        iota(v.begin(), v.end(), 0);
        do{
            for(int i=0; i<p.size()-1; i++){
                sum += distance(p[v[i]], p[v[i+1]]);
            }
        }while(next_permutation(v.begin(), v.end()));

        double fact = 1;
        for(int i=1; i<=n; i++){
            fact *= i;
        }

        double res = sum / fact;
        out << fixed << setprecision(10) << res << endl;
	}
};


int main() {
    cin.tie(nullptr);
    ios_base::sync_with_stdio(false);
	CAverageLength solver;
	std::istream& in(std::cin);
	std::ostream& out(std::cout);
	solver.solve(in, out);
	return 0;
}


Measured memory_bytes: 256