#include <bits/stdc++.h>
using namespace std;
using ll = long long;
const int INF = 1e9 + 10;
const ll INFL = 4e18;

/*
    考察
    P:=(i,j)(i<j)であって、二次元平面でjがiの右上にあるような組の個数
    Q:=(i,j)(i<j)であって、二次元平面でjがiの右下にあるような組の個数
    ->座標圧縮して平面走査かな
*/

template <typename T>
struct FenwickTree {
    FenwickTree() = default;
    FenwickTree(int n) {
        this->n = n;
        dat = vector<T>(n);
    }
    void add(int i, T x) {
        i++;
        while (i <= n) {
            dat[i - 1] += x;
            i += i & -i;
        }
    }
    T sum(int l, int r) {
        return sum(r) - sum(l);
    }
    T operator[](int i) {
        return sum(i, i + 1);
    }
    int size() {
        return n;
    }

private:
    int n;
    vector<T> dat;
    T sum(int r) {
        T ret = 0;
        while (r > 0) {
            ret += dat[r - 1];
            r -= r & -r;
        }
        return ret;
    }
};

int main() {
    int N;
    cin >> N;
    vector<int> val;
    vector<pair<int, int>> P(N);
    for (int i = 0; i < N; i++) {
        cin >> P[i].first >> P[i].second;
        val.push_back(P[i].first);
        val.push_back(P[i].second);
    }

    ranges::sort(val);
    val.erase(unique(val.begin(), val.end()), val.end());

    auto get = [&](ll x) {
        return ranges::lower_bound(val, x) - val.begin();
    };

    vector<vector<int>> YS(N * 2);
    for (int i = 0; i < N; i++) YS[get(P[i].first)].push_back(get(P[i].second));

    FenwickTree<ll> ft(N * 2);
    ll cup = 0, cdw = 0;

    for (int i = N * 2 - 1; i >= 0; i--) {
        for (int y : YS[i]) {
            cup += ft.sum(y + 1, 2 * N);
            cdw += ft.sum(0, y);
        }
        for (int y : YS[i]) ft.add(y, 1);
    }

    vector<int> cntx(N * 2), cnty(N * 2);
    for (int i = 0; i < N; i++) {
        cntx[get(P[i].first)]++;
        cnty[get(P[i].second)]++;
    }

    ll cx = 0, cy = 0;
    for (int i = 0; i < N * 2; i++) {
        cx += 1ll * (N - cntx[i]) * cntx[i];
        cy += 1ll * (N - cnty[i]) * cnty[i];
    }
    cx /= 2;
    cy /= 2;

    double ans = 1.0 * (cup - cdw) / sqrt(cx * cy);
    printf("%.10lf\n", ans);
}