#include <bits/stdc++.h>
using namespace std;
using ll=long long;
using ull=unsigned long long;
using pll=pair<ll,ll>;
using tll=tuple<ll,ll,ll>;
using ld=long double;
const ll INF=(1ll<<60);
#define rep(i,n) for (ll i=0;i<(ll)(n);i++)
#define replr(i,l,r) for (ll i=(ll)(l);i<(ll)(r);i++)
#define all(v) v.begin(),v.end()
#define len(v) ((ll)v.size())
template<class T> inline bool chmin(T &a,T b){
    if(a>b){
        a=b;
        return true;
    }
    return false;
}
template<class T> inline bool chmax(T &a,T b){
    if(a<b){
        a=b;
        return true;
    }
    return false;
}
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
template<class T> using super_set=__gnu_pbds::tree<T,__gnu_pbds::null_type,std::less<T>,__gnu_pbds::rb_tree_tag,__gnu_pbds::tree_order_statistics_node_update>;
ll f(vector<pll> v){
    super_set<tll> stx;
    super_set<pll> sty;
    ll ret=0;
    rep(i,len(v)){
        ret+=sty.order_of_key({v[i].second,-INF});
        ret-=stx.order_of_key({v[i].first,v[i].second,-INF})-stx.order_of_key({v[i].first,-INF,-INF});
        stx.insert({v[i].first,v[i].second,i});
        sty.insert({v[i].second,i});
    }
    return ret;
}
int main(){
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    ll n;
    cin >> n;
    vector<ll> x(n),y(n);
    vector<pll> v(n);
    rep(i,n){
        cin >> x[i] >> y[i];
        v[i]={x[i],y[i]};
    }
    sort(all(v));
    ll p=f(v);
    rep(i,n){
        v[i].first*=-1;
    }
    sort(all(v));
    ll q=f(v);
    
    map<ll,ll> cx,cy;
    ll r=0,s=0;
    rep(i,n){
        r+=i-cx[x[i]];
        cx[x[i]]++;
        s+=i-cy[y[i]];
        cy[y[i]]++;
    }
    cout << fixed << setprecision(10) <<  (ld)(p-q)/sqrtl((ld)r*s) << '\n';
}