using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.IO;
using static System.Console;
using static System.Math;
 
public class Solve{
    static public int mod = 1000000007;
    static public string al = "abcdefghijklmnopqrstuvwxyz";
    public static void Main(){
        // 方針
        //
        var n = rint();
        var x = new double[n];
        var y = new double[n];
        for(int i=0;i<n;i++){
            var d = doublea();
            x[i] = d[0];
            y[i] = d[1];
        }
        var per = new Permutation(n);
        
        var ans = 0.0;
        while(true){
            var p = per.now();
            for(int i=1;i<n;i++){
                var nx = x[p[i]] - x[p[i-1]];
                var ny = y[p[i]] - y[p[i-1]];
                ans += Sqrt(nx*nx + ny*ny);
            }
            
            if(!per.next()){
                break;
            }
        }
        
        var divide = 1.0;
        for(int i=2;i<=n;i++){
            divide *= i;
        }
        WriteLine(ans/divide);
        
	
	
    }
    public static void swap(ref int a,ref int b){int temp = a;a= b;b = temp;}
    static void charswap(ref char a,ref char b){char temp = a;a= b;b = temp;}
    static int ncr(int n,int r){if(n<r)return 0;r = Min(r,n-r);long nn = 1;for(int i=n-r+1;i<=n;i++){nn = nn*i%mod;}long rr = 1;for(int i=1;i<=r;i++){rr = rr*i%mod;}rr = square((int)rr,mod-2);nn = nn * rr %mod;return (int)nn;}
    // a^b mod
    static int square(int a,int b){string binary = Convert.ToString(b,2);int bileng = binary.Length;long a_power = a;long value = 1;for(int i=bileng-1;i>=0;i--){if(binary[i] == '1'){value = value*a_power%mod;}a_power = a_power*a_power%mod;}return (int)value;}
    static int square2(int a,int b){long output = 1;var list = new List<long>();int sh = 1;long n = a;list.Add(a);while(sh < b){sh *= 2;n = n*n%mod;list.Add(n);}for(int i=list.Count-1;i>=0;i--){if(b > sh){b -= sh;sh /= 2;output = output*list[i]%mod;}}return (int)output;}
    //各種読取
    static string rstr(){ return ReadLine(); }
    static int rint(){ return int.Parse(ReadLine()); }
    static long rlong(){ return long.Parse(ReadLine()); }
    static string[] stra(){ return ReadLine().Split(' '); }
    static char[] chara(){ string[] a=stra();string b="";for(int i=0;i<a.Length;i++){b+=a[i];}return b.ToCharArray();}
    static int[] inta(){ string[] read_str_array = ReadLine().Split(' '); int[] return_int_array = new int[read_str_array.Length]; for(int countup_i=0;countup_i<read_str_array.Length;countup_i++){ return_int_array[countup_i] = int.Parse(read_str_array[countup_i]); } return return_int_array; }
    static int[,] inta2(int num_array,int in_array){ int[,] int_array2 = new int[num_array,in_array]; for(int i=0;i<num_array;i++){ int[] temp_array = inta(); for(int j=0;j<in_array;j++){ int_array2[i,j] = temp_array[j]; } } return int_array2; }
    static long[] longa(){ string[] read_str_array = ReadLine().Split(' '); long[] return_long_array = new long[read_str_array.Length]; for(long countup_i=0;countup_i<read_str_array.Length;countup_i++){ return_long_array[countup_i] = long.Parse(read_str_array[countup_i]); } return return_long_array; }
    static double[] doublea(){ string[] read_str_array = ReadLine().Split(' '); double[] return_double_array = new double[read_str_array.Length]; for(long countup_i=0;countup_i<read_str_array.Length;countup_i++){ return_double_array[countup_i] = double.Parse(read_str_array[countup_i]); } return return_double_array; }
    // -----------------------------
    static long GCD(long a,long b){ if(a < b){ long temp = a; a = b; b = temp; } if(a % b == 0){ return b; } else{ long temp = b; b = a%b; a = temp; return GCD(a,b); } }
    static long LCM(long a,long b){ return a * b / GCD(a,b); }
    static void WriteArray(int[,] a,int b,int c){for(int i=0;i<b;i++){for(int j=0;j<c;j++){if(j!=0) Write(" ");Write(a[i,j]);}WriteLine();}}
}
class Permutation{
    private int[] _permutation;
    private int _length;
    
    public Permutation(int a){
        _permutation = new int[a];
        for(int i=0;i<a;i++){
            _permutation[i] = i;
        }
        _length = a;
    }
    public int[] now(){
        return _permutation;
    }
    
    public bool next(){
        var upper = -1;
        for(int i=0;i<_length-1;i++){
            if(_permutation[i] < _permutation[i+1]) upper = i;
        }
        if(upper == -1){
            return false;
        }
        var bigger = -1;
        for(int i=upper;i<_length;i++){
            if(_permutation[upper] <= _permutation[i]) bigger = i;
        }
        var temp = _permutation[upper];
        _permutation[upper] = _permutation[bigger];
        _permutation[bigger] = temp;
        var list = new List<int>();
        upper++;
        for(int i=upper;i<_length;i++){
            list.Add(_permutation[i]);
        }
        list.Reverse();
        for(int i=upper;i<_length;i++){
            _permutation[i] = list[i-upper];
        }
        return true;
    }
    
    public void prev0(){
        Array.Reverse(_permutation);
    }
    
    public bool prev(){
        var lower = -1;
        for(int i=0;i<_length-1;i++){
            if(_permutation[i] > _permutation[i+1]) lower = i;
        }
        if(lower == -1){
            return false;
        }
        var smaller = -1;
        for(int i=lower;i<_length;i++){
            if(_permutation[lower] >= _permutation[i]) smaller = i;
        }
        var temp = _permutation[lower];
        _permutation[lower] = _permutation[smaller];
        _permutation[smaller] = temp;
        var list = new List<int>();
        lower++;
        for(int i=lower;i<_length;i++){
            list.Add(_permutation[i]);
        }
        list.Reverse();
        for(int i=lower;i<_length;i++){
            _permutation[i] = list[i-lower];
        }
        return true;
    }
}

Measured memory_bytes: 30124