#include <stdio.h>
#include <vector>
#include <cmath>



using namespace std;

struct Point{
	int x;
	int y;

};

double size(double x, double y){
	return sqrt((x * x) + (y * y));
}

double PI = acos(-1);

int main(void){
	
	int N;
	
	while(scanf("%d ", &N) == 1){
		vector<struct Point> mesta(N);
		for(int i = 0; i < N; i++){
			int x, y;
			scanf("%d %d ", &x, &y);
			Point mesto;
			mesto.x = x;
			mesto.y = y;
			mesta[i] = mesto;
		}
		double c = 0;
		double total = N*N;
		for(int i = 0; i < mesta.size(); i++){
			for(int i1 = i + 1; i1 < mesta.size(); i1++){
				int v_x = abs(mesta[i].x - mesta[i1].x);
				int v_y = abs(mesta[i].y - mesta[i1].y);
				
				if(v_x == v_y)
					++c;
				
			}
		}
		printf("%lf\n", (c*2) / total);
	
	}
	return 0;
}