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

using namespace std;

typedef long long ll;

int main(void){
	ll n;
	while(scanf("%lld", &n) == 1){
		ll data[123456];
		ll data2[123456];
		for(int i = 0; i < n; i++){
			ll x,y;
			scanf("%lld %lld", &x, &y);
			data[i] = y-x;
			data2[i] = y+x;
		}
		sort(data,data+n);
		sort(data2,data2+n);
		int i = 0;
		ll l = 1;
		ll l2 = 1;
		ll sumg = 0;
		while(i < n-1){
			if(data[i] != data[i+1]){
				sumg += l*(l-1);
				l = 1;
			}
			else
				l+=1;
			if(data2[i] != data2[i+1]){
				sumg += l2*(l2-1);
				l2 = 1;
			}
			else
				l2+=1;
			i++;
		}
		sumg += l*(l-1);
		sumg += l2*(l2-1);
		//printf("%lld %lld\n", sumg, n*n);
		printf("%.12lf\n", ((double)sumg)/((double)n*n));
	}
	return 0;
}