#include <stdio.h>
#include <vector>
#include <cmath>
#include <map>



using namespace std;

//vector<int> mesta_x(100066);
//vector<int> mesta_y(100066);
//vector<int> sum(100066);

long long pocet(long long n) {

	return (n*(n-1))/2;
}
		
int main(void){
	
	int N;
	
	while(scanf("%d ", &N) == 1){
		map<int, int> m1;
		map<int, int> m2;
		
		double total = N*N;
		for(int i = 0; i < N; i++){
			int x, y;
			scanf("%d %d ", &x, &y);
			
			long long c1 = y + x;
			long long c2 = y - x;
			
			m1[c1]++;
			m2[c2]++;		
}
	long long suma = 0;
	for (map<int, int>::iterator it = m1.begin(); it != m1.end(); ++it) {
		suma += pocet(it->second);	
	} 
	for (map<int, int>::iterator it = m2.begin(); it != m2.end(); ++it) {
		suma += pocet(it->second);	
	} 
	
		
	printf("%lf\n", (2*suma) / total);
	
	}
	
	return 0;
}