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

using namespace std;

int main() {
	int n;
	while (scanf("%d", &n) > 0) {
		map<int, int> dia1;
		map<int, int> dia2;
		for (int i = 0; i < n; i++) {
			int x, y;
			scanf("%d %d", &x, &y);
			dia1[x + y] += 1;
			dia2[y - x] += 1;
		}

		double prob = 0;
		for (map<int, int>::const_iterator it = dia1.begin(); it != dia1.end(); it++) {
			//printf("dia1: %d -> %d\n", it->first, it->second);
			if (it->second > 1) {
				double p1 = (double)(it->second) / (double)n;
				double p2 = (double)(it->second - 1) / (double)n;
				prob += p1 * p2;
			}
		}
		for (map<int, int>::const_iterator it = dia2.begin(); it != dia2.end(); it++) {
			//printf("dia2: %d -> %d\n", it->first, it->second);			
			if (it->second > 1) {
				double p1 = (double)(it->second) / (double)n;
				double p2 = (double)(it->second - 1) / (double)n;
				prob += p1 * p2;
			}
		}

		printf("%lf\n", prob);
	}

	return 0;
}