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

using namespace std;

int fce(const void *x, const void *y)
{
	return *(long long int*)x < *(long long int*)y;
}
int main(){
	int n;
	while(scanf("%d", &n)+1){
		long long int p1[n];
		long long int p2[n];
		for(int i = 0; i < n; i++){
			int a, b;
			scanf("%d %d", &a, &b);
			p1[i] = b - a;
			p2[i] = a + b;
		}
		qsort(p1, n, sizeof(long long int), fce);
		qsort(p2, n, sizeof(long long int), fce);

		long long int count = 0;
		long long 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/(double)(n*n));
	}
	return 0;
}