#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define ALL(v) v.begin(),v.end()
#define dbg(x) cerr << #x << ": " << (x) << endl;
template<class F, class S>
ostream& operator<<(ostream& os, pair<F,S>& p) {
    os << '(' << p.first << ',' << p.second << ')';
    return os;
}
template<class Iter>
void print(Iter beg, Iter end) {
    for (Iter itr = beg; itr != end; ++itr) {
        cerr << *itr << ' ';
    }
    cerr << '\n';
}
class BIT {
public:
    BIT(int n) 
    : n(n), d(n+1,0) {
    }
    // index番目にvalを加える
    void add(int index, ll val) {
        assert(0 <= index && index < n);
        ++index;
        for (int x = index; x <= n; x += x & -x) {
            d[x] += val;
        }
    }
    // [0, index)の総和
    ll sum(int index) {
        ll ret = 0;
        for (int x = index; x > 0; x -= x & -x) {
            ret += d[x];
        }
        return ret;
    }
    // [l, r)の総和
    ll sum(int l, int r) {
        l = max(0, l);
        r = min(r, n);
        return sum(r) - sum(l);
    }
private:
    int n;
    vector<ll> d;
};
int n;
vector<ll> x, y;
int main() {
    cin >> n;
    x.resize(n);
    y.resize(n);
    for (int i = 0; i < n; ++i) cin >> x[i] >> y[i];

    vector<int> ord(n);
    iota(ALL(ord), 0);
    ll p = 0, q=0, r=0, s=0;
    {
        sort(ALL(ord), [](int a, int b) {
            return y[a] < y[b];
        });
        ll now = -1e18;
        int cnt = 0;
        for (int i : ord) {
            if (y[i] == now) {
                ++cnt;
            } else {
                q += ll(n - cnt) * cnt;
                now = y[i];
                cnt = 1;
            }
        }
        q += ll(n - cnt) * cnt;
        assert(q % 2 == 0);
        q /= 2;
        dbg(q);
    }
    {
        sort(ALL(ord), [](int a, int b) {
            return x[a] < x[b];
        });
        ll now = -1e18;
        int cnt = 0;
        for (int i : ord) {
            if (x[i] == now) {
                ++cnt;
            } else {
                p += cnt * ll(n - cnt);
                now = x[i];
                cnt = 1;
            }
        }
        p += cnt * ll(n - cnt);
        assert(p % 2 == 0);
        p /= 2;
        dbg(p);
    }
    {
        auto tmp = y;
        sort(ALL(tmp));
        tmp.erase(unique(ALL(tmp)), tmp.end());
        for (int i = 0; i < n; ++i) y[i] = lower_bound(ALL(tmp), y[i]) - tmp.begin();
        BIT bit(tmp.size());
        vector<int> add;
        ll now = 1e18;
        for (int i : ord) {
            if (x[i] == now) {
                add.push_back(y[i]);
            } else {
                for (int j : add) {
                    bit.add(j, 1);
                }
                add.clear();
                now = x[i];
                add.push_back(y[i]);
            }
            r += bit.sum(0, y[i]);
            s += bit.sum(y[i]+1, tmp.size());
        }
        dbg(r)
        dbg(s)
    }
    double ans = (r - s) / (sqrt(p) * sqrt(q));
    cout << fixed << setprecision(20) << ans << '\n';
}