#include<bits/stdc++.h>
namespace {
#pragma GCC diagnostic ignored "-Wunused-function"
#include<atcoder/all>
#pragma GCC diagnostic warning "-Wunused-function"
using namespace std;
using namespace atcoder;
#define rep(i,n) for(int i = 0; i < (int)(n); i++)
#define rrep(i,n) for(int i = (int)(n) - 1; i >= 0; i--)
#define all(x) begin(x), end(x)
#define rall(x) rbegin(x), rend(x)
template<class T> bool chmax(T& a, const T& b) { if (a < b) { a = b; return true; } else return false; }
template<class T> bool chmin(T& a, const T& b) { if (b < a) { a = b; return true; } else return false; }
using ll = long long;
using P = pair<int,int>;
using VI = vector<int>;
using VVI = vector<VI>;
using VL = vector<ll>;
using VVL = vector<VL>;

template <class S>
struct value_compression : vector<S> {
  bool built = false;
  using VS = vector<S>;
  using VS::VS;
  value_compression(vector<S> v) : vector<S>(move(v)) {}
  void build() {
    sort(this->begin(), this->end());
    this->erase(unique(this->begin(), this->end()), this->end());
    built = true;
  }
  template <class T>
  void convert(T first, T last) {
    assert(built);
    for (; first != last; ++first) *first = (*this)(*first);
  }
  int operator()(S x) {
    assert(built);
    return lower_bound(this->begin(), this->end(), x) - this->begin();
  }
  void clear() {
    this->clear();
    built = false;
  }
};

} int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  int n;
  cin >> n;
  vector<P> xy(n);
  value_compression<int> vc_x, vc_y;
  for (auto& [x, y] : xy) cin >> x >> y, vc_x.emplace_back(x), vc_y.emplace_back(y);
  vc_x.build();
  vc_y.build();
  for (auto& [x, y] : xy) x = vc_x(x), y = vc_y(y);
  ll p = 0, q = 0, r = n * ll(n - 1) / 2, s = r;
  ranges::sort(xy, {}, [](const P& p) { return P(p.first, -p.second); });
  fenwick_tree<int> ft(n);
  for (auto [x, y] : xy) p += ft.sum(0, y), ft.add(y, 1);
  ranges::sort(xy, {}, [](const P& p) { return P(-p.first, -p.second); });
  ft = fenwick_tree<int>(n);
  for (auto [x, y] : xy) q += ft.sum(0, y), ft.add(y, 1);
  VI cntx(n), cnty(n);
  for (auto [x, y] : xy) r -= cntx[x]++, s -= cnty[y]++;
  auto res = (p - q) / sqrtl(__int128(r) * s);
  cout << fixed << setprecision(16) << res << '\n';
}