#include <bits/stdc++.h>
using namespace std;
using std::cout;
using std::cin;
using std::endl;
using ll=long long;
using ld=long double;
const ll ILL=2167167167167167167;
const int INF=2100000000;
#define rep(i,a,b) for (int i=(int)(a);i<(int)(b);i++)
#define all(p) p.begin(),p.end()
template<class T> using _pq = priority_queue<T, vector<T>, greater<T>>;
template<class T> ll LB(vector<T> &v,T a){return lower_bound(v.begin(),v.end(),a)-v.begin();}
template<class T> ll UB(vector<T> &v,T a){return upper_bound(v.begin(),v.end(),a)-v.begin();}
template<class T> bool chmin(T &a,T b){if(a>b){a=b;return 1;}else return 0;}
template<class T> bool chmax(T &a,T b){if(a<b){a=b;return 1;}else return 0;}
template<class T> void So(vector<T> &v) {sort(v.begin(),v.end());}
template<class T> void Sore(vector<T> &v) {sort(v.begin(),v.end(),[](T x,T y){return x>y;});}
bool yneos(bool a,bool upp=0){if(a){cout<<(upp?"YES\n":"Yes\n");}else{cout<<(upp?"NO\n":"No\n");}return a;}
template<class T> void vec_out(vector<T> &p,int ty=0){
    if(ty==2){cout<<'{';for(int i=0;i<(int)p.size();i++){if(i){cout<<",";}cout<<'"'<<p[i]<<'"';}cout<<"}\n";}
    else{if(ty==1){cout<<p.size()<<"\n";}for(int i=0;i<(int)(p.size());i++){if(i) cout<<" ";cout<<p[i];}cout<<"\n";}}
template<class T> T vec_min(vector<T> &a){assert(!a.empty());T ans=a[0];for(auto &x:a) chmin(ans,x);return ans;}
template<class T> T vec_max(vector<T> &a){assert(!a.empty());T ans=a[0];for(auto &x:a) chmax(ans,x);return ans;}
template<class T> T vec_sum(vector<T> &a){T ans=T(0);for(auto &x:a) ans+=x;return ans;}
int pop_count(long long a){int res=0;while(a){res+=(a&1),a>>=1;}return res;}
template<class T> bool inside(T l,T x,T r){return l<=x&&x<r;}

#include <atcoder/segtree>

int op(int a, int b){
    return a + b;
}
int e(){
    return 0;
}

void solve();
// CYAN / FREDERIC
int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    int t = 1;
    // cin >> t;
    rep(i, 0, t) solve();
}

void solve(){
    ll P = 0, Q = 0, R = 0, S = 0;
    ll N;
    cin >> N;
    vector<pair<int, int>> p(N);
    set<int> s;
    rep(i, 0, N){
        cin >> p[i].first >> p[i].second;
        s.insert(p[i].first);
        s.insert(p[i].second);
    }
    map<int, int> m;
    for (auto x : s) m[x] = (int)m.size();
    rep(i, 0, N){
        p[i].first = m[p[i].first];
        p[i].second = m[p[i].second];
    }
    int M = m.size();
    m.clear();
    rep(rp, 0, 2){
        R = (N * (N - 1)) / 2;
        rep(i, 0, N){
            R -= m[p[i].first];
            m[p[i].first]++;
            swap(p[i].first, p[i].second);
        }
        m.clear();
        swap(R, S);
    }
    vector<vector<int>> G(M);
    rep(i, 0, N) G[p[i].first].push_back(i);
    atcoder::segtree<int, op, e> seg(M);
    for (auto x : G){
        for (auto y : x){
            Q += seg.prod(p[y].second + 1, M);
            P += seg.prod(0, p[y].second);
        }
        for (auto y : x){
            auto tmp = seg.get(p[y].second);
            seg.set(p[y].second, tmp + 1);
        }
    }
    //cout << P << " " << Q << " " << R << " " << S << "\n";
    cout << setprecision(20) << (ld)(P - Q) / sqrtl(R * S) << "\n";
}