#include <stdio.h>
#include <bits/stdc++.h>
#include <atcoder/all>
using namespace atcoder;
using mint = modint998244353;
using namespace std;
#define rep(i,n) for (int i = 0; i < (n); ++i)
#define Inf32 1000000001
#define Inf64 1000000000000000001
long long get(vector<long long> x,vector<long long> y){
	vector<long long> t = y;
	sort(t.begin(),t.end());
	t.erase(unique(t.begin(),t.end()),t.end());
	vector<pair<long long,long long>> p;
	rep(i,x.size()){
		p.emplace_back(x[i],lower_bound(t.begin(),t.end(),y[i])-t.begin());
	}
	sort(p.begin(),p.end());
	fenwick_tree<long long> fw(t.size());
	long long ret = 0;
	rep(i,p.size()){
		ret += fw.sum(p[i].second+1,t.size());
		fw.add(p[i].second,1);
	}
	return ret;
}
long long g2(vector<long long> x){
	long long n = x.size();
	long long ret = n*(n-1)/2;
	map<long long,long long> mp;
	rep(i,n){
		ret -= mp[x[i]];
		mp[x[i]]++;
	}
	return ret;
}
int main(){
	
	int n;
	cin>>n;
	vector<long long> x(n),y(n);
	rep(i,n){
		cin>>x[i]>>y[i];
	}
	long long P = get(x,y);
	rep(i,n){
		x[i] *= -1;
	}
	long long Q = get(x,y);
	long long R = g2(x),S = g2(y);
	long double ans = P-Q;
	ans /= sqrtl(R);
	ans /= sqrtl(S);
	ans *= -1;
	cout<<fixed<<setprecision(15)<<ans<<endl;
    return 0;
}