#include <iostream>
#include <locale>
#include <bits/stdc++.h>
using namespace std;


struct point {
   int x;
   int y;
};

int main(int argc, char** argv) {

    std::ios::sync_with_stdio(false);
    int m;
    while(cin >> m){
        vector <point> arr;
        for(int i =0; i < m ;++i){
            int x,y;
            point p;
            cin >> x >> y;
            p.x = x;
            p.y = y;
            arr.push_back(p);
        }
        
        
        int matchCnt = 0;
        
        for(int i = 0; i < m; ++i){
            for(int j = 0; j < m; ++j){
            
                
                if(arr[i].x == arr[j].x  && arr[i].y == arr[j].y) continue;
                if( abs(arr[i].x - arr[j].x) == abs(arr[i].y - arr[j].y) ){
                    matchCnt++;
                }
            }
        }
       
        
        
        
        cout << (float)matchCnt/((float)m*m) << endl;
        
        
      
        
    }
    
    return 0;
}