#include <bits/stdc++.h>
#include <atcoder/all>
using namespace std;
using namespace atcoder;
#define rep(i, n) for(int i=0;i<(n);++i)
#define rep1(i, n) for(int i=1;i<=(n);i++)
#define ll long long
using mint = modint998244353;
using P = pair<ll,ll>;
using lb = long double;
using T = tuple<ll, ll, ll>;
#ifdef LOCAL
#  include <debug_print.hpp>
#  define dbg(...) debug_print::multi_print(#__VA_ARGS__, __VA_ARGS__)
#else
#  define dbg(...) (static_cast<void>(0))
#endif

// Coodinate Compression
// https://youtu.be/fR3W5IcBGLQ?t=8550
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();
  }
};

int main()
{
    ll n;
    cin >> n;
    vector<pair<int, int>> vs(n);
    rep(i,n) cin >> vs[i].first >> vs[i].second;
    sort(vs.begin(),vs.end());
    CC cx, cy;
    rep(i,n) cy.add(vs[i].second);
    int sy = cy.size();
    fenwick_tree<int> bit(sy);
    ll p = 0;
    ll q = 0;
    for(int i=0;i<n;) {
        int j = i;
        vector<int> y;
        while(j<n && vs[j].first==vs[i].first) {
            y.push_back(cy(vs[j].second));
            j++;
        }
        for(int Y : y) {
            dbg(Y);
            p += bit.sum(0, Y);    
            q += bit.sum(Y+1, sy);
        }  
        for(int Y : y) {
            bit.add(Y,1);
        }
        i = j;
    }
    map<int,int> x, y;
    ll r = n * (n-1)/2;
    ll s = n * (n-1)/2;
    rep(i,n) x[vs[i].first]++;
    rep(i,n) y[vs[i].second]++;
    for(auto p : x) {
        r -= (ll)p.second*(p.second-1)/2;
    }
    for(auto p : y) {
        s -= (ll)p.second*(p.second-1)/2;
    }
    dbg(p,q,r,s);
    lb ans = (p-q) / sqrt(r*s);
    cout << fixed << setprecision(16) << ans << endl;
    return 0;
}