#include <stdio.h>
#include <vector>
#include <algorithm>

using namespace std;

vector<pair<int,int> > points;

int main(){
	int n;
	while(scanf("%d", &n)+1){
		points.resize(n);
		vector<int> p1 = vector<int>(n);
		vector<int> p2 = vector<int>(n);
		for(int i = 0; i < n; i++){
			scanf("%d %d", &points[i].first, &points[i].second);
			p1[i] = points[i].second - points[i].first;
			p2[i] = points[i].first + points[i].second;
		}
		sort(p1.begin(), p1.end());
		sort(p2.begin(), p2.end());

		/*printf("Sorted:\n");
		for(int i = 0; i < n; i++){
			printf("%d %d\n", p1[i], p2[i]);
		}*/


		int count = 0;
		int tmp = 1;
		for(int i = 1; i < n; i++){
			if(p1[i-1] == p1[i]){
				tmp++;
			} else {
				count += tmp*(tmp-1);
				tmp = 1;
			}
		}
		count += tmp*(tmp-1);
		tmp = 1;
		for(int i = 1; i < n; i++){
			if(p2[i-1] == p2[i]){
				tmp++;
			} else {
				count += tmp*(tmp-1);
				tmp = 1;
			}
		}
		count += tmp*(tmp-1);
		printf("%lf\n", (double)count/(n*n));
		points.clear();
	}
	return 0;
}