/*
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*/
#include "bits/stdc++.h"
using namespace std;
#define ok1 printf("ok1\n")
#define ok2 printf("ok2\n")
#define M 1000000000000000000LL
#define rep(i,n) for(int i=0;i<n;++i)
#define REP(i,s,n) for(int i=(s);i<(n);++i)
#define repr(i,n) for(int i=n-1;i>=0;--i)
#define REPR(i,s,n) for(int i=(s);i>=(n);--(i))
#define all(a) (a).begin(),(a).end()
#define reall(a) (a).rbegin(),(a).rend()
#define pb emplace_back //emplace_backの方が速いが使い慣れてないため
#define DOUBLE fixed << setprecision(15)
#define fi first
#define se second
#define mp make_pair
#define mt make_tuple
#define BIT(n,m)(((n)>>(m))&1)
const double pi = acos(-1.0);
typedef vector<int> vi;
typedef vector<string> vs;
typedef long long ll;
typedef vector<ll> vll;
typedef vector<vi> vvi;
typedef vector<vll> vvll;
typedef vector<char> vc;
typedef vector<double> vd;
typedef vector<bool> vb;
typedef deque<ll> dll;
typedef pair<ll, ll> P;
typedef vector<P> vP;
//const ll mod = 1e9 + 7;
const ll mod = 998244353;
ll dy[4] = { 1,0,-1,0 };
ll dx[4] = { 0,1,0,-1 };
template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return true; } return false; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return true; } return false; }
/*
template< typename T >
using Edges = vector< edge< T > >;
template< typename T >
using WeightedGraph = vector< Edges< T > >;
using UnWeightedGraph = vector< vector< int > >;
template< typename T >
using Matrix = vector< vector< T > >;
*/
bool out_check(ll a, ll b) { return (0 <= a && a < b); }
void addmod(ll &a, ll &b) { a = (a + b) % mod; }
void Pvi(vi v) {
	puts("vll------------------------------------------------");
	rep(i, v.size()) cout << v[i] << " "; puts("");
	puts("------------------------------------------------");
}
void Pvll(vll v) {
	puts("vll------------------------------------------------");
	rep(i, v.size()) cout << v[i] << " "; puts("");
	puts("------------------------------------------------");
}
void Pvvll(vvll v) {
	puts("vvll------------------------------------------------");
	rep(i, v.size()) { rep(j, v[i].size()) cout << v[i][j] << " "; cout << endl; }
	puts("------------------------------------------------");
}
void Pvs(vs s) {
	puts("vs------------------------------------------------");
	rep(i, s.size()) { rep(j, s[i].size()) cout << s[i][j] << " "; cout << endl; }
	puts("------------------------------------------------");
}

void Yes(bool x) { cout << ((x) ? "Yes\n" : "No\n"); }
void YES(bool x) { cout << ((x) ? "YES\n" : "NO\n"); }
void yes(bool x) { cout << ((x) ? "yes\n" : "no\n"); }
void Yay(bool x) { cout << ((x) ? "Yay!\n" : ":(\n"); }

ll  n, m, d, e, r, l, k, h, ans, ret = M;
bool flag = false, flag2 = false, flag3 = false;
string s, t, u;
vll x, y;

int main() {
	cin.tie(0); ios::sync_with_stdio(false);//not intractive
	cin >> n;
	x.resize(n), y.resize(n);
	rep(i, n) {
		cin >> x[i] >> y[i];
	}
	vll index(n);
	rep(i, n)index[i] = i;
	double sum = 0,a=0;
	ll cnt = 0;
	do {
		cnt++;
		REP(i, 1, n) {
			sum += sqrt((x[index[i - 1]] - x[index[i]])*(x[index[i - 1]] - x[index[i]]) + (y[index[i - 1]] - y[index[i]])*(y[index[i - 1]] - y[index[i]]));
		}
		//cout << DOUBLE<<sum - a << endl;
	} while (next_permutation(all(index)));
	cout << DOUBLE << sum / cnt << endl;
	return 0;
}


Measured memory_bytes: 256