/*
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 */

import java.io.*;
import java.io.BufferedReader;
import java.io.InputStreamReader;

/**
 *
 * @author cteam056
 */

// předělat na Main
class Main {

    public static BufferedReader in; 
    private static String line;
    private static int[][] monsters;
    private static String [][] strMonsters;
    private static int N;
    /**
     * @param args the command line arguments
     * @throws java.io.IOException
     */
    public static void main(String[] args) throws IOException {
        // TODO code application logic here
        in = new BufferedReader(new InputStreamReader(System.in));
        
        while((line = in.readLine()) != null && !line.isEmpty()) 
        {
            N = Integer.parseInt(line);
            monsters = new int[N][2];
            strMonsters = new String[N][2];
            
            for(int j = 0; j < N; j++)
            {
                line = in.readLine();
                strMonsters[j] = line.split(" ");

                for(int i = 0; i < 2; i++)
                {
                    monsters[j][i] = Integer.parseInt(strMonsters[j][i]);
                }
            }
            
            
            
            System.out.println(seradMonstra());
        }
    }
    
    private static double seradMonstra()
    {
        double p = 0;
        
        int a, b;
        
        for(int i = 0; i < N; i++)
        {
            for (int j = i + 1; j < N; j++)
            {
                a = monsters[i][0] - monsters[j][0];
                b = monsters[i][1] - monsters[j][1];
                if (myAbs(a) == myAbs(b))
                {
                    p += 2;
                }
            }
        }
        
        return p / (N * N);
    }
    
    private static int myAbs(int a)
    {
        return (a < 0) ? a * -1 : a;
    }
}