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

import java.util.Random;
import java.util.Scanner;

/**
 *
 * @author cteam037
 */
public class Northwest {

    void run() {
        Scanner sc = new Scanner(System.in);
        while (sc.hasNext()) {
            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(new Random().nextDouble());
            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;
        }
    }
}