#include <string>
#include <iostream>
#include <vector>
#include <unordered_map>

int main() {
	using namespace std;
	ios::sync_with_stdio(false);
	
	size_t n = 0;
	while (cin >> n) {
		unordered_map<size_t, size_t> sz{};
		unordered_map<size_t, size_t> sv{};
		for (size_t i = 0; i < n; ++i) {
			int a,b;
			cin >> a >> b;
			sz[a - b + n] += 1;
			sv[a+b] += 1;
		}
		size_t c = 0;
		for (auto i : sz) {
			if (i.second > 1) {
				c += i.second * (i.second - 1);
			}
		}

		for (auto i : sv) {
			if (i.second > 1) {
				c += i.second * (i.second -1);
			}
		}
		double res = (double) c / ((double) n * (double) n);
		cout << res << endl;
	}

			
	return 0;
}