#include <atcoder/fenwicktree>
#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)

void solve() {
  ll n;
  cin >> n;
  vector<pair<ll, ll>> xy(n);
  rep(i, n) cin >> xy[i].first >> xy[i].second;
  sort(xy.begin(), xy.end());
  vector<ll> pre(n, -1), nex(n, 1);
  rep(i, n) {
    while (xy[pre[i] + 1].first < xy[i].first)
      pre[i]++;
    while (nex[i] < n && xy[i].first == xy[nex[i]].first)
      nex[i]++;
    if (i < n - 1)
      pre[i + 1] = pre[i], nex[i + 1] = nex[i];
  }
  priority_queue<pair<ll, ll>> que;
  map<ll, vector<ll>> yis;
  atcoder::fenwick_tree<ll> ft(n);
  rep(i, n) {
    ll y = xy[i].second;
    que.emplace(y, i);
    yis[y].push_back(i);
  }
  ll p = 0, q = 0, last_y = 1e18;
  while (!que.empty()) {
    auto [y, i] = que.top();
    que.pop();
    if (y < last_y) {
      for (const auto j : yis[last_y])
        ft.add(j, 1);
      last_y = y;
    }
    if (pre[i] >= 0) {
      q += ft.sum(0, pre[i] + 1);
    }
    if (nex[i] <= n - 1) {
      p += ft.sum(nex[i], n);
    }
  }
  ll r = 0, s = 0;
  {
    ll rem = n;
    map<ll, ll> cnt;
    rep(i, n) cnt[xy[i].first]++;
    for (const auto [k, v] : cnt)
      r += v * (rem - v), rem -= v;
  }
  {
    ll rem = n;
    map<ll, ll> cnt;
    rep(i, n) cnt[xy[i].second]++;
    for (const auto [k, v] : cnt)
      s += v * (rem - v), rem -= v;
  }
  double ans = p - q;
  ans /= sqrt(double(r) * double(s));
  cout << fixed << setprecision(16) << ans << '\n';
}

int main() {
  std::cin.tie(nullptr);
  std::ios_base::sync_with_stdio(false);
  int T = 1;
  for (int t = 0; t < T; t++) {
    solve();
  }
  return 0;
}