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

int main(){
    ios::sync_with_stdio(false);
    cin.tie(0);
    int n;
    cin >> n;
    ll P = 0, R = 0, S = 0, cn = 0;
    vector<pair<int,int>> a(n);
    vector<int> cy(n);
    for(int i = 0; i < n; i++){
        auto &&[x, y] = a[i];
        cin >> x >> y;
        cy[i] = y;
    }
    sort(a.begin(), a.end());
    sort(cy.begin(), cy.end());
    cy.erase(unique(cy.begin(), cy.end()), cy.end());
    const int m = cy.size();
    atcoder::fenwick_tree<int> fw(m);
    vector<int> cnty(m);
    for(int l = 0, r = 0; l < n; l = r){
        while(r < n && a[r].first == a[l].first) r++;
        R += cn * (r - l);
        for(int i = l; i < r; i++){
            a[i].second = lower_bound(cy.begin(), cy.end(), a[i].second) - cy.begin();
            int low = fw.sum(0, a[i].second);
            int up = cn - cnty[a[i].second] - low;
            P += low - up;
        }
        for(int i = l; i < r; i++){
            cn++, cnty[a[i].second]++;
            S += cn - cnty[a[i].second];
            fw.add(a[i].second, 1);
        }
    }
    cout << fixed << setprecision(15) << P / sqrtl((double)R * S) << '\n';
}