#include <iostream>
#include <map>

using namespace std;

//ios::sync_with_stdio(false);

int main()
{
    unsigned n = 0;

	while (cin >> n) {
		map<long int, unsigned> dia1;
		map<long int, unsigned> dia2;
		for (unsigned i=0; i<n; ++i) {
			long int x, y;
			cin >> x;
			cin >> y;
			dia1[y+x] += 1;
			dia2[y-x] += 1;
		}
		unsigned ok = 0;
		for (auto i=dia1.begin(); i!=dia1.end(); ++i) {
			ok += i->second*i->second - i->second;
		}
		for (auto i=dia2.begin(); i!=dia2.end(); ++i) {
			ok += i->second*i->second - i->second;
		}
		cout << ok/float(n*n) << endl;
	}
}