#include <bits/stdc++.h>

using namespace std;
using lli = int64_t;

int main () {
	for (;;) {
		lli n;
		cin >> n;
		if (!cin) break;
		auto choose2 = [] (lli a) { return a*(a-1)/2; };
		
		auto nw = map<lli, lli> {};
		auto ne = map<lli, lli> {};

		for (lli i = 0; i < n; i++) {
			lli x, y;
			cin >> x >> y;
			nw[x+y]++;
			ne[x-y]++;
		}

		lli sum = 0;

		for (auto & v : nw)
			sum += choose2(v.second);
		for (auto & v : ne)
			sum += choose2(v.second);
		sum *= 2;

		auto r = sum / (long double) (n * n);

		cout << fixed << setprecision (10) << r << '\n';
	}
	return 0;
}