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

void fast_io() {
	ios_base::sync_with_stdio(false);
	cin.tie(nullptr);
}

#include <atcoder/fenwicktree>
using namespace atcoder;
int main() {
	fast_io();
	int n;
	cin >> n;
	vector<pair<int, int>> xy(n);
	vector<int> vals;
	for (int i = 0; i < n; i++) {
		cin >> xy[i].first >> xy[i].second;
		vals.push_back(xy[i].first);
		vals.push_back(xy[i].second);
	}
	sort(vals.begin(), vals.end());
	vals.erase(unique(vals.begin(), vals.end()), vals.end());
	int m = vals.size();
	for (int i = 0; i < n; i++) {
		xy[i].first =
			lower_bound(vals.begin(), vals.end(), xy[i].first) - vals.begin();
		xy[i].second =
			lower_bound(vals.begin(), vals.end(), xy[i].second) - vals.begin();
	}
	long long r = (long long)n * (n - 1) / 2, s = r;
	{
		vector<int> cnt_x(m), cnt_y(m);
		for (auto [x, y] : xy) {
			r -= cnt_x[x];
			cnt_x[x]++;
			s -= cnt_y[y];
			cnt_y[y]++;
		}
	}
	long long p = 0;
	{
		sort(xy.begin(), xy.end(), [&](auto a, auto b) {
			if (a.first == b.first) return a.second > b.second;
			return a.first < b.first;
		});
		fenwick_tree<long long> fw(m);
		for (auto [_, y] : xy) {
			p += fw.sum(0, y);
			fw.add(y, 1);
		}
	}
	long long q = 0;
	{
		sort(xy.begin(), xy.end(), [&](auto a, auto b) {
			if (a.first == b.first) return a.second < b.second;
			return a.first < b.first;
		});
		fenwick_tree<long long> fw(m);
		for (auto [_, y] : xy) {
			q += fw.sum(y + 1, m);
			fw.add(y, 1);
		}
	}
	long double ans = (p - q) / sqrtl(r) / sqrtl(s);
	cout << fixed << setprecision(16) << ans << endl;
}