import java.util.Scanner;


public class Northwest {

	/**
	 * @param args
	 */
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);

		while (sc.hasNext()) {
			int pocet = sc.nextInt();
			
			
			int[] x = new int[pocet];
			int[] y = new int[pocet];
			
			for (int i = 0; i < pocet; i++) {
				x[i] = sc.nextInt();
				y[i] = sc.nextInt();
			}
			
			int diag = 0;
		
			for (int i = 0; i < pocet; i++) {
				for (int j = 0; j < pocet; j++) {
					if ((Math.abs(x[i] - x[j]) == Math.abs(y[i] - y[j]))) {
						diag++;
					}
				}
			}
			int celkem = pocet*pocet;
			diag = diag - pocet;

			System.out.println(diag/(double)celkem);
			
		}

	}

}