import java.util.HashMap;
import java.util.Map;
import java.util.Scanner;

/**
 *
 * @author muran4
 */
public class GoNorth {

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        Scanner S = new Scanner(System.in);
        HashMap<Integer,Integer> nw,ne;
        nw = new HashMap<>();
        ne = new HashMap<>();
        int n,x,y;
        int arr[][];
        Integer u=0;
        while(S.hasNextInt()){
            n=S.nextInt();
            arr=new int[n][2];
            nw.clear();
                ne.clear();
            for (int i = 0; i < n; i++) {
                
                x=S.nextInt();
                y=S.nextInt();
                arr[i][0]=x;
                arr[i][1]=y;
                u=nw.get(x+y);
                if(u==null){
                    nw.put(x+y, 1);
                }else{
                    nw.put(x+y, u+1);
                }
                
                u=ne.get(y-x);
                if(u==null){
                    ne.put(y-x, 1);
                }else{
                    ne.put(y-x, u+1);
                }
                
            }
                            double res=0;
             for (Map.Entry<Integer, Integer> entry : ne.entrySet()) {
                Integer key = entry.getValue();
                 //System.out.println(key+"ne");
                res+=((key*(key-1)));
                
            }
             for (Map.Entry<Integer, Integer> entry : nw.entrySet()) {
                Integer key = entry.getValue();
               // System.out.println(key+"nw");
                res+=((key*(key-1)));
                
            }
                
                //System.out.println(res+" --");
                
                System.out.format("%.6f%n",(res*1.0)/(n*n));
        }
    }
    
    /*static double res(int x){
        double res=0,det;
        det=Math.sqrt(1+4*2*x);
        System.out.println("det"+((1+det)/2));
        return ((1+det)/2);
    }*/
    
}