#include <iostream>
#include <vector>
#include <map>
#include <cmath>
#include <algorithm>

using namespace std;

template <typename T> class segTree
{
private:
  const T e;
  int num;
  std::vector<T> dat;
  T (*const eval)(T &, T &) {};
public:
  segTree(std::vector<T> &v, T E, T (*func)(T &, T &)) : e(E), eval(func)
  {
    int siz = static_cast<int>(v.size());
    for (num = 1; num < siz; num <<= 1);
    dat = std::vector<T> (2 * num - 1, e);
    for (int i = 0; i < siz; ++i) dat[i + num - 1] = v[i];
    for (int i = num - 2; i >= 0; --i) dat[i] = eval(dat[i * 2 + 1], dat[i * 2 + 2]);
  }
  segTree(int n, T E, T (*func)(T &, T &)) : e(E), eval(func)
  {
    for (num = 1; num < n; num <<= 1);
    dat = std::vector<T> (2 * num - 1, e);
  }
  void update_a(int i, T val)
  {
    for (i += num - 1, dat[i] = val; i != 0;)
    {
      i = (i - 1) / 2;
      dat[i] = eval(dat[i * 2 + 1], dat[i * 2 + 2]);
    }
  }
  void update_r(int i, T val)
  {
    for (i += num - 1, dat[i] = eval(dat[i], val); i != 0;)
    {
      i = (i - 1) / 2;
      dat[i] = eval(dat[i * 2 + 1], dat[i * 2 + 2]);
    }
  }
  T getval(int left, int right)
  {
    left = max(0, left), right = min(num, right);
    T ansl = e, ansr = e;
    for (left += num - 1, right += num - 1; left < right; left >>= 1, right >>= 1)
    {
      if (!(left & 1)) ansl = eval(ansl, dat[left]);
      if (--right & 1) ansr = eval(dat[right], ansr);
    }
    return eval(ansl, ansr);
  }
  T getval(int id) {return dat[num - 1 + id];}
};

int main()
{
  int n;
  cin >> n;
  vector<int> x(n), y(n);
  for (int i = 0; i < n; ++i) cin >> x[i] >> y[i];
  auto cal = [&](vector<int> &a) -> long long
  {
    long long ret = 0;
    map<int, int> mp;
    for (int i = 0; i < n; ++i) ++mp[a[i]];
    for (int i = 0; i < n; ++i) ret += n - mp[a[i]];
    return ret / 2;
  };
  long long r = cal(x), s = cal(y);
  vector<pair<int, int>> xy(n);
  auto cal2 = [&]() -> long long
  {
    vector<int> all;
    for (auto [a, b] : xy) all.push_back(b);
    sort(all.begin(), all.end());
    all.erase(unique(all.begin(), all.end()), all.end());
    int siz = all.size();
    segTree<int> sg(siz, 0, [](int &a, int &b) -> int {return a + b;});
    long long ret = 0;
    for (auto [a, b] : xy)
    {
      int id = lower_bound(all.begin(), all.end(), b) - all.begin();
      ret += sg.getval(0, id);
      sg.update_r(id, 1);
    }
    return ret;
  };
  for (int i = 0; i < n; ++i) xy[i] = {x[i], -y[i]};
  sort(xy.begin(), xy.end());
  for (int i = 0; i < n; ++i) xy[i].second *= -1;
  long long p = cal2();
  for (int i = 0; i < n; ++i) xy[i] = {-x[i], -y[i]};
  sort(xy.begin(), xy.end());
  for (int i = 0; i < n; ++i) xy[i].second *= -1;
  long long q = cal2();
  double ans = (p - q) / sqrt((double)r * (double)s);
  printf("%.10f\n", ans);
}