/*
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 * To change this template file, choose Tools | Templates
 * and open the template in the editor.
 */


import java.util.Scanner;

/**
 *
 * @author cteam037
 */
public class Northwest {
    void run(){
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        Town[] t = new Town[n];
        for(int i = 0;i<n;i++){
            t[i]= new Town(sc.nextInt(), sc.nextInt());
        }
        double sum = n*n;
        double counterValid = 0;
        for(int i = 0;i<n;i++) {
            for(int j = 0;j<n;j++){
                int a = t[i].x - t[j].x;
                int b = t[i].y - t[j].y;
                if(a == b && b == 0){
                    continue;
                }
                if((a==b && b>0)||(a==-b && b > 0)){
                    counterValid ++;
                }
            }
        }
        System.out.println(counterValid*2/sum);
    }
    public static void main(String[] args) {
        new Northwest().run();
    }
    
    class Town{
        int x;
        int y;
        Town(int x, int y){
            this.x = x;
            this.y = y;
        }
    }
}