#include <iostream>
#include <string>
#include <algorithm>
#include <cassert>
#include <limits>
#include <numeric>
#include <type_traits>
#include <vector>
#include <list>
#include <map>
#include <set>
#include <deque>
#include <stack>
#include <bitset>
#include <functional>
#include <utility>
#include <sstream>
#include <iomanip>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <cctype>
#include <string>
#include <cstring>
#include <ctime>
#include <complex>
#include <climits>
#include <time.h>

using namespace std;
typedef long long ll;
typedef vector<ll> vl;
typedef vector<string> vs;
typedef pair<ll, ll> P;
#define FOR(i,a,b) for(ll i=(a);i<(b);i++)
#define REP(i,n)  FOR(i,0,n)

ll gcd(ll a, ll b) {
  while (a && b) {
    if (a >= b) a %= b;
    else b %= a;
  }
  return a + b;
}
ll ketasum(ll n) { //桁の数字全ての和を返す 例: 112 -> 4
	ll sum=0;
	if (n<0) return 0;
	while (n>0) {
		sum += n%10;
		n /= 10;
	}
	return sum;
}

typedef pair<ll, ll> pair_t;

bool comp(const pair_t &a, const pair_t &b) {
        return a.first < b.first;
}

void sort2vectors(vector<ll> &av, vector<ll> &bv)
{ // ふたつの配列をまとめてそーと A[2,1] B[3,4] -> 1,4 2,3
        ll n = av.size();
        vector<pair_t> p(n);
        vector<ll> av2(n), bv2(n);
        for (ll i = 0; i < n; i++)
                p[i] = make_pair(av[i], bv[i]);
        sort(p.begin(), p.end(), comp);
        for (ll i = 0; i < n; i++) {
                av2[i] = p[i].first;
                bv2[i] = p[i].second;
        }
        av = av2;
        bv = bv2;
}
ll facctorialMethod(ll k){ //階乗
	ll s;
	if(k > 1){
		s = k*facctorialMethod(k-1);
	} else {
		s = 1;
	}
	return s;
}

map< int64_t, ll > prime_factor(int64_t n) {
  map< int64_t, ll > ret;
  for(int64_t i = 2; i * i <= n; i++) {
    while(n % i == 0) {
      ret[i]++;
      n /= i;
    }
  }
  if(n != 1) ret[n] = 1;
  return ret;
}
// dp[i] := 長さが i の増加部分列として最後尾の要素のとりうる最小値
template<class T> int LIS(vector<T> a,  bool is_strong = true) {
    const T INF = 1<<30; // to be set appropriately
    int n = (int)a.size();
    vector<T> dp(n, INF);
    for (int i = 0; i < n; ++i) {
        if (is_strong) *lower_bound(dp.begin(), dp.end(), a[i]) = a[i];
        else *upper_bound(dp.begin(), dp.end(), a[i]) = a[i];
    }
    return lower_bound(dp.begin(), dp.end(), INF) - dp.begin();
}
long long modpow(long long a, long long n, long long mod) {
    long long res = 1;
    while (n > 0) {
        if (n & 1) res = res * a % mod;
        a = a * a % mod;
        n >>= 1;
    }
    return res;
}
void foreach_permutation(ll n, std::function<void(ll *)> f) {
  ll indexes[n];
  for (ll i = 0; i < n; i++) indexes[i] = i;
  do {
    f(indexes);
  } while (std::next_permutation(indexes, indexes + n));
}

const ll mod = 1000000007;
const ll INF = 1e9;

// 変数定義start(vectorはresizeしようね)
ll N;
vl x;
vl y;
// 変数定義end
long double dist(ll x, ll y, ll x2, ll y2){
	return sqrt((x-x2)*(x-x2) + (y-y2)*(y-y2));
}
void solve(){
	cin >> N;
	x.resize(N);
	y.resize(N);
	REP(i, N){
		cin >> x[i] >> y[i];
	}
	long double ans = 0.0;
	foreach_permutation(N, [&](ll *indexes){
		REP(i, N-1){
			ans += dist(x[indexes[i]], y[indexes[i]], x[indexes[i+1]], y[indexes[i+1]]);
		}
	});
	cout << ans / facctorialMethod(N) << endl;

}

int main(){

    //clock_t start = clock();

    cout << std::fixed << std::setprecision(10);
	ios::sync_with_stdio(false);
	cin.tie(nullptr);
	solve();
	//clock_t end = clock();

    //const double time = static_cast<double>(end - start) / CLOCKS_PER_SEC * 1000.0;
    //printf("time %lf[ms]\n", time);
}


Measured memory_bytes: 764