#include<bits/stdc++.h>
using namespace std;

#define pb emplace_back
#define mp make_pair
 
using ll = long long;
using pii = pair<int,int>;
 
constexpr int mod = 998244353;
constexpr int inf = 0x3f3f3f3f;
constexpr int N = 2e5 + 10;

int n, m, b[N];
int s[N];

int lowbit(int x){
  return x & (-x);
}
void add(int x, int v){
  for(; x<=m; x+=lowbit(x)) s[x] += v;
}
int sum(int x){
  int ret = 0;
  for(; x>0; x-=lowbit(x))  ret += s[x];
  return ret;
}

int query(int L, int R){
  if(L > R) return 0;
  return sum(R) - sum(L - 1);
}

void _main(){
  unordered_map<int, int> cntx, cnty;
  map<int, vector<int>> g;
  cin >> n;
  ll P = 0, Q = 0, R = 0, S = 0;
  int x, y;
  m = 0;
  for(int i=1; i<=n; ++i){
    cin >> x >> y;
    R += (i - ++cntx[x]);
    S += (i - ++cnty[y]);
    g[x].pb(y);
    b[m++] = y;
  }
  sort(b, b+m);
  m = unique(b, b+m) - b;
  for(auto &[x, vec] : g){
    for(int y : vec){
      int j = lower_bound(b, b+m, y) - b + 1;
      P += query(1, j - 1);
      Q += query(j + 1, m);
    }
    for(int y : vec){
      int j = lower_bound(b, b+m, y) - b + 1;
      add(j, 1);
    }
  }
  cout << setprecision(8) << (P - Q) * 1.0 / sqrt(1.0 * R * S) << '\n';
}

int main(){
  ios::sync_with_stdio(0);
  cin.tie(0); cout.tie(0);
  _main();
  return 0;
}