#include<bits/stdc++.h>
using namespace std;
int N;
int points[100005][2];
int dist[100005][2];
double P;
int a0, b0, c0;
int a1, b1, c1;
int m1, m2;
map<int,int> map0;
map<int,int> map1;
int main(){
	a0 = 1;
	b0 = 1;
	c0 = 0;
	a1 = 1;
	b1 = -1;
	c1 = 0;
	while(scanf("%d", &N) && !feof(stdin)){
		memset(points,0,sizeof(points));
		P = 0;
		map0.clear();
		map1.clear();
		for(int i = 0;i<N;++i){
			scanf("%d %d", &m1, &m2);
			dist[i][0] = a0*m1+b0*m2;
			dist[i][1] = a1*m1+b1*m2;
			auto it0 = map0.find(dist[i][0]);
			if(it0 == map0.end()){
				map0.insert({dist[i][0],1});
			} else {
				(it0->second)++;
			}
			auto it1 = map1.find(dist[i][1]);
			if(it1 == map1.end()){
				map1.insert({dist[i][1],1});
			} else {
				(it1->second)++;
			}
		}

		for(int i = 0;i<N;++i){
			auto it0 = map0.find(dist[i][0]);
			auto it1 = map1.find(dist[i][1]);
			int cnt0 = it0->second;
			int cnt1 = it1->second;
			double pi = (double)(cnt0+cnt1-2)/(N);
			P+=pi;
		}

		P/=N;

		printf("%g\n",P); 
	}
	return 0;
}