#include <iostream>
#include <cmath>

using namespace std;

struct Mesto {
	int x, y;
};

int main(void) {

	while(true) {
		int pocet;
		cin >> pocet;
		
		if(cin.eof()) break;

		Mesto * pole = new Mesto[pocet]; 
		
		for(int i = 0; i < pocet; i++) {
			cin >> pole[i].x >> pole[i].y;
		}
		
		int pocitadlo = 0;
		for(int i = 0; i < pocet; i++) {
			for(int j = 0; j < pocet; j++) {
				if(i == j) continue;
				if(fabs(pole[i].x - pole[j].x) == fabs(pole[i].y - pole[j].y)) pocitadlo++;
			}
		}
		
		double vysledek = (double)pocitadlo / (double)(pocet*pocet);

		cout << vysledek << endl;
		
		delete [] pole;
	}
	
	return 0;
}