#include <bits/stdc++.h>
#include <atcoder/fenwicktree>
using namespace std;
using namespace atcoder;
using ll = long long;

int main() {
    int N;
    cin >> N;

    vector<pair<int, int>> p(N);
    for (int i = 0; i < N; i++) cin >> p[i].first >> p[i].second;

    // x座標を圧縮
    vector<int> xs;
    for (int i = 0; i < N; i++) xs.push_back(p[i].first);
    sort(xs.begin(), xs.end());
    xs.erase(unique(xs.begin(), xs.end()), xs.end());
    for (int i = 0; i < N; i++) p[i].first = lower_bound(xs.begin(), xs.end(), p[i].first) - xs.begin();

    // y座標を圧縮
    vector<int> ys;
    for (int i = 0; i < N; i++) ys.push_back(p[i].second);
    sort(ys.begin(), ys.end());
    ys.erase(unique(ys.begin(), ys.end()), ys.end());
    for (int i = 0; i < N; i++) p[i].second = lower_bound(ys.begin(), ys.end(), p[i].second) - ys.begin();

    // x座標でソート
    sort(p.begin(), p.end());

    // x毎にy座標を管理
    vector<vector<int>> v(xs.size());
    for (int i = 0; i < N; i++) v[p[i].first].push_back(p[i].second);

    // BITでP,Q,R,Sを求める
    fenwick_tree<ll> fw(ys.size());
    ll P, Q, R, S;
    P = Q = 0, R = S = (ll)N * (N - 1) / 2;
    for (int i = 0; i < xs.size(); i++) {
        for (auto y: v[i]) {
            P += fw.sum(0, y);
            Q += fw.sum(y + 1, ys.size());
        }
        for (auto y: v[i]) fw.add(y, 1);
        R -= (ll)v[i].size() * (v[i].size() - 1) / 2;
    }
    for (int i = 0; i < ys.size(); i++) {
        ll cnt = fw.sum(i, i+1);
        S -= cnt * (cnt - 1) / 2;
    }

    cout << fixed << setprecision(16) << (double)(P-Q)/sqrt(R)/sqrt(S) << endl;
}