/*
 * To change this license header, choose License Headers in Project Properties.
 * To change this template file, choose Tools | Templates
 * and open the template in the editor.
 */
package northwest;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

/**
 *
 * @author hlavaty7
 */
public class Main {

    public static void main(String[] args) throws IOException {
        class Cor {

            public Cor(int x, int y) {
                this.x = x;
                this.y = y;
            }

            int x;
            int y;
            // int bol;
        }

        BufferedReader sc = new BufferedReader(new InputStreamReader(System.in));

        int p;
        Cor[] cors;
        String line;
        while ((line = sc.readLine()) != null) {

            p = Integer.parseInt(line.trim());

            cors = new Cor[p];

            for (int i = 0; i < p; i++) {
                String[] pom = sc.readLine().split(" ");
                cors[i] = new Cor(Integer.parseInt(pom[0]), Integer.parseInt(pom[1]));

            }

            double vysl = 0;

            for (int i = 0; i < p; i++) {
                for (int j = 0; j < p; j++) {
                    if (i != j) {
                        if ((cors[i].x < cors[j].x && cors[i].y < cors[j].y) || (cors[i].x > cors[j].x && cors[i].y < cors[j].y)) {
                            int sumX = Math.abs(cors[i].x - cors[j].x);
                            int sumY = Math.abs(cors[i].y - cors[j].y);
                            if (sumX == sumY) {

                                vysl = vysl + 2;
                            }
                        }
//                        if ((cors[i].x > cors[j].x && cors[i].y > cors[j].y) || (cors[i].x < cors[j].x && cors[i].y > cors[j].y)) {
//                            int sumX = Math.abs(cors[i].x - cors[j].x);
//                            int sumY = Math.abs(cors[i].y - cors[j].y);
//                            if (sumX == sumY) {
//                                
//                                vysl++;
//                            }
//                        }
                    }
                }
            }
            System.out.println(((vysl) / ((double) (p * p))));
        }
    }
}