#include <iostream>
#include <map>

using namespace std;

int main() {
	long n;

	while (cin >> n) {
		map<long, long> mapa1;
		map<long, long> mapa2;
		long long options = n*n, winningOptions = 0;
		long x, y;

		for (long i = 0; i < n; i++) {
			cin >> x >> y;

			long diff = x - y;
			long summ = x + y;

			if (mapa1.find(diff) != mapa1.end()) {
				mapa1[diff]++;
			}
			else {
				mapa1[diff] = 1;
			}

			if (mapa2.find(summ) != mapa2.end()) {
				mapa2[summ]++;
			}
			else {
				mapa2[summ] = 1;
			}
		}

		for (map<long, long>::iterator it = mapa1.begin(); it != mapa1.end(); ++it) {
			if (it->second > 1) {
				winningOptions += (it->second * (it->second - 1));
			}
		}
		for (map<long, long>::iterator it = mapa2.begin(); it != mapa2.end(); ++it) {
			if (it->second > 1) {
				winningOptions += (it->second * (it->second - 1));
			}
		}

		double result = (double)winningOptions / (double)options;

		cout << result << endl;
		
	}

	return 0;
}