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

using SS = int;
class SegmentTree{
    public:
    int siz = 1,n;
    vector<SS> dat;
 
    SS op(SS a, SS b){return a+b;}
    SS e(){return 0;}
    void renew (SS &a,SS x){
        a = op(a,x);
        //a = x;
        //その他.
    }
 
    void make(int N){
        while(siz < N) siz *= 2;
        n = N; dat.resize(siz*2,e());
    }
 
    void make2(int N,vector<SS> &A){
        make(N);
        for(int i=0; i<N; i++){
            int pos = i+siz;
            dat.at(pos) = A.at(i);
        }
        for(int i=siz-1; i>0; i--) dat.at(i) = op(dat.at(i*2),dat.at(i*2+1));
    }
 
    void update(int pos,SS x){
        pos = pos+siz;
        renew(dat.at(pos),x);
        while(pos != 1){
            pos = pos/2;
            dat.at(pos) = op(dat.at(pos*2),dat.at(pos*2+1));
        }
    }
 
    SS findans(int l, int r){
        SS retl = e(),retr = e();
        l += siz,r += siz;
        while(l < r){
            if(l&1) retl = op(retl,dat.at(l++));
            if(r&1) retr = op(dat.at(--r),retr);
            l >>= 1; r >>= 1;
        }
        return op(retl,retr);
    }
 
    SS get(int pos){return dat.at(pos+siz);}
    SS rangeans(int l, int r){return findans(l,r);}
    SS allrange(){return dat.at(1);}
 
    //rightは) leftは[で 渡す&返す. 
    int maxright(const function<bool(SS)> f,int l = 0){
        l += siz; int r = n+siz;
        vector<int> ls,rs;
        while(l < r){
            if(l&1) ls.push_back(l++);
            if(r&1) rs.push_back(--r);
            l >>= 1; r >>= 1; 
        }
        SS okl = e();
        for(int i=0; i<ls.size(); i++){
            l = ls.at(i);
            SS now = op(okl,dat.at(l));
            if(!f(now)){
                while(l < siz){
                    l <<= 1;
                    now = op(okl,dat.at(l));
                    if(f(now)){okl = now; l++;}
                }
                return l-siz;
            } 
            okl = now;
        }
        for(int i=rs.size()-1; i>=0; i--){
            l = rs.at(i);
            SS now = op(okl,dat.at(l));
            if(!f(now)){
                while(l < siz){
                    l <<= 1;
                    now = op(okl,dat.at(l));
                    if(f(now)){okl = now; l++;}
                }
                return l-siz;
            } 
            okl = now;
        }
        return n;
    }
    int minleft(const function<bool(SS)> f,int r = -1){
        if(r == -1) r = n;
        int l = siz; r += siz;
        vector<int> ls,rs;
        while(l < r){
            if(l&1) ls.push_back(l++);
            if(r&1) rs.push_back(--r);
            l >>= 1; r >>= 1; 
        }
        SS okr = e();
        for(int i=0; i<rs.size(); i++){
            r = rs.at(i);
            SS now = op(dat.at(r),okr);
            if(!f(now)){
                while(r < siz){
                    r <<= 1; r++;
                    now = op(dat.at(r),okr);
                    if(f(now)){okr = now; r--;}
                }
                return r+1-siz;
            }
        }
        for(int i=ls.size()-1; i>=0; i--){
            r = ls.at(i);
            SS now = op(dat.at(r),okr);
            if(!f(now)){
                while(r < siz){
                    r <<= 1; r++;
                    now = op(dat.at(r),okr);
                    if(f(now)){okr = now; r--;}
                }
                return r+1-siz;
            }
        }
        return 0;
    }
    //ノーマルセグ木.一年の時を経て漸く二分探索実装した.
};

int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);

    long long N; cin >> N;    
    vector<pair<int,int>> XY(N);
    vector<int> all;
    for(auto &[x,y] : XY) cin >> x >> y,all.push_back(x),all.push_back(y);
    sort(all.begin(),all.end());
    all.erase(unique(all.begin(),all.end()),all.end());
    vector<long long> Xs(2*N),Ys(2*N);
    map<pair<int,int>,long long> XYs;
    for(auto &[x,y] : XY){
        x = lower_bound(all.begin(),all.end(),x)-all.begin();
        y = lower_bound(all.begin(),all.end(),y)-all.begin();
        Xs.at(x)++; Ys.at(y)++; XYs[{x,y}]++;
    }

    long long P = 0,Q = 0,R = 0,S = 0,ex = 0,pairs = N*(N-1)/2;
    for(auto &[k,v] : XYs) ex += v*(v-1)/2;
    for(auto &v : Xs) R += v*(v-1)/2;
    for(auto &v : Ys) S += v*(v-1)/2;
    sort(XY.begin(),XY.end());
    int back = -1;
    vector<int> memo;
    SegmentTree Z; Z.make(2*N);
    for(auto &[x,y] : XY){
        if(x != back){
            for(auto &p : memo) Z.update(p,1);
            memo.clear();
        }
        back = x;
        P += Z.rangeans(0,y); memo.push_back(y);
    }
    Q = pairs-P-R-S+ex;
    R = pairs-R; S = pairs-S;

    cout << fixed << setprecision(20) << (P-Q+(long double)0.0)/sqrtl((long double)R*S) << endl;
}