#include<bits/stdc++.h>
#include<atcoder/fenwicktree>
using namespace std;
int N,X[2<<17],Y[2<<17];
int main()
{
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    cin>>N;
    map<int,vector<int>>mpX,mpY;
    for(int i=0;i<N;i++)
    {
        cin>>X[i]>>Y[i];
        mpX[X[i]].push_back(Y[i]);
        mpY[Y[i]].push_back(X[i]);
    }
    vector<int>ord(N);
    vector<int> B(Y,Y+N);
    sort(B.begin(),B.end());
    B.erase(unique(B.begin(),B.end()),B.end());
    atcoder::fenwick_tree<int>BIT(B.size());
    long P=0,Q=0;
    for(auto[key,val]:mpX)
    {
        for(int y:val)
        {
            y=lower_bound(B.begin(),B.end(),y)-B.begin();
            P+=BIT.sum(0,y);
            Q+=BIT.sum(y+1,B.size());
        }
        for(int y:val)
        {
            y=lower_bound(B.begin(),B.end(),y)-B.begin();
            BIT.add(y,1);
        }
    }
    long R=long(N)*(N-1)/2;
    long S=R;
    for(auto[key,val]:mpX)
    {
        long x=val.size();
        R-=x*(x-1)/2;
    }
    for(auto[key,val]:mpY)
    {
        long x=val.size();
        S-=x*(x-1)/2;
    }
    double ans=P-Q;
    ans/=sqrtl(double(R)*S);
    cout<<fixed<<setprecision(16)<<ans<<endl;
}