import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.util.ArrayList;
import java.util.HashMap;

/*
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/**
 *
 * @author cteam014
 */
public class Northwest {
    BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
    
    
    public void solve() throws  Exception {
        String line;
        while ((line = reader.readLine()) != null) {
            long N = Integer.parseInt(line);
            //int[] towns = new int[N*2];
            
            HashMap<Long, Integer> indicesNW = new HashMap();
            HashMap<Long, Integer> indicesNE = new HashMap();
            
            ArrayList<Integer> diagonalsNW = new ArrayList<>(10000);
            ArrayList<Integer> diagonalsNE = new ArrayList<>(10000);
            
            
            for (int i = 0; i < N; i++) {
                line = reader.readLine();
                String[] a = line.split(" ");
                int x = Integer.parseInt(a[0]);
                int y = Integer.parseInt(a[1]);
                
                long NWdiagonal = x + y;
                long NEdiagonal = y - x;
                
                if (indicesNW.containsKey(NWdiagonal)) {
                    int index = indicesNW.get(NWdiagonal);
                    diagonalsNW.set(index, diagonalsNW.get(index) + 1);
                } else {
                    indicesNW.put(NWdiagonal, diagonalsNW.size());
                    diagonalsNW.add(1);
                }
                
                if (indicesNE.containsKey(NEdiagonal)) {
                    int index = indicesNE.get(NEdiagonal);
                    diagonalsNE.set(index, diagonalsNE.get(index) + 1);
                } else {
                    indicesNE.put(NEdiagonal, diagonalsNE.size());
                    diagonalsNE.add(1);
                }
                
            }
            
            long numOfDiagonalPairs = 0;
            for (int i = 0; i <  diagonalsNW.size(); i++) {
                long diagonal = diagonalsNW.get(i);
                numOfDiagonalPairs += diagonal * (diagonal - 1);
            }
            for (int i = 0; i <  diagonalsNE.size(); i++) {
                long diagonal = diagonalsNE.get(i);
                numOfDiagonalPairs += diagonal * (diagonal - 1);
            }
            
            float probability = ((float) numOfDiagonalPairs) / (N*N);
            System.out.println(probability);
        }   
    }
    
    public static void main(String[] args) throws Exception {
        new Northwest().solve();
        //long a = 100000;
        //System.out.println(a*a);
    }
}