#include <string>
#include <iostream>
#include <map>

using namespace std;

map<int, int> R;
map<int, int> L;

int main(){
	int N;
	int sum;
	while(cin >> N){
	
		//cout << "XXXX" << endl;
		L.clear();
		R.clear();
		
		for(int i = 0; i < N; i++){
			int x, y;
			cin >> x >> y;
			R[x + y]++;
			L[x - y]++;
			//cout << "a " << R.size() << endl;
		}
		
		sum = 0;
		
		for(auto& v : L){
			int x = v.second;
			sum += x * (x-1);
			//cout << "L " << v.first << ' ' << v.second << endl; 
		}
		
		for(auto& v : R){
			int x = v.second;
			sum += x * (x-1);
			//cout << "R " << v.first << ' ' << v.second << endl; 
		}
		
		double S = N;
		cout << (double)sum / (S * S) << endl;
	}
	return 0;
}