#include <atcoder/all>
#include <bits/stdc++.h>
#define rep(i, a, b) for (ll i = (ll)(a); i < (ll)(b); i++)
using namespace atcoder;
using namespace std;

typedef long long ll;

template <typename T = long long> struct CC {
    bool initialized;
    vector<T> xs;
    CC() : initialized(false) {}
    void add(T x) { xs.push_back(x); }
    void init() {
        sort(xs.begin(), xs.end());
        xs.erase(unique(xs.begin(), xs.end()), xs.end());
        initialized = true;
    }
    int operator()(T x) {
        if (!initialized)
            init();
        return upper_bound(xs.begin(), xs.end(), x) - xs.begin() - 1;
    }
    T operator[](int i) {
        if (!initialized)
            init();
        return xs[i];
    }
    int size() {
        if (!initialized)
            init();
        return xs.size();
    }
};

ll f(int n, vector<int> x, vector<int> y) {
    ll ret = 0;
    CC<int> cc;
    rep(i, 0, n) { cc.add(y[i]); }
    rep(i, 0, n) { y[i] = cc(y[i]); }
    vector<pair<int, int>> p(n);
    rep(i, 0, n) p[i] = {x[i], -y[i]};
    sort(p.begin(), p.end());
    fenwick_tree<int> fw(n);
    for (auto [x, y] : p) {
        y *= -1;
        ret += fw.sum(0, y);
        fw.add(y, 1);
    }
    return ret;
}

ll g(int n, vector<int> x) {
    ll ret = 0;
    map<int, int> ma;
    rep(i, 0, n) { ma[x[i]]++; }
    rep(i, 0, n) { ret += n - ma[x[i]]; }
    ret /= 2;
    return ret;
}

int main() {
    int n;
    cin >> n;
    vector<int> x(n), y(n);
    rep(i, 0, n) cin >> x[i] >> y[i];

    vector<int> xm(n), ym(n);
    rep(i, 0, n) {
        xm[i] = -x[i];
        ym[i] = -y[i];
    }
    double P = f(n, x, y) + f(n, xm, ym);
    rep(i, 0, n) { x[i] *= -1; }
    double Q = f(n, x, y) + f(n, xm, ym);
    rep(i, 0, n) { x[i] *= -1; }
    double R = g(n, x);
    double S = g(n, y);
    cerr << P << " " << Q << " " << R << " " << S << endl;
    double ans = (P - Q) / sqrt(R * S);
    cout << fixed << setprecision(15) << ans << endl;
}