#include "bits/stdc++.h"
using namespace std;

int n;
vector<vector<int>> mp(8, vector<int>(2));
int tot = 0;

void ave_len(int _now_num, vector<int> mati, vector<int> nokori) {
	if (_now_num == n) {
		for (int i = 1; i < n; i++) {
			tot += sqrt((mp[mati[i]][0] - mp[mati[i - 1]][0]) * (mp[mati[i]][0] - mp[mati[i - 1]][0]) + (mp[mati[i]][1] - mp[mati[i]][1]) * (mp[mati[i]][1] - mp[mati[i]][1]));
		}
	}

	for (int i = _now_num; i < n; i++) {
		mati[_now_num] = nokori[i];
		nokori.erase(nokori.begin() + i);
		ave_len(_now_num + 1, mati, nokori);
	}
}

int kaijou(int j) {
	int num = 1;
	for (int i = 1; i <= j; i++) {
		num *= i;
	}

	return num;
}

int main() {
	int tot = 0;
	cin >> n;

	vector<int> map(n);
	vector<int> number(n);

	for (int i = 0; i < n; i++) {
		number[i] = i;
	}

	for (int i = 0; i < n; i++)
		cin >> mp[i][0] >> mp[i][1];

	ave_len(0, map, number);

	double ans;
	double kai;
	kai = kaijou(n);
	ans = tot / kai;
	cout << ans << endl;
	
	return 0;
}