#include <iostream>
#include <locale>
#include <bits/stdc++.h>

using namespace std;


class point {
public:
    point(int ax, int ay): x(ax),y(ay){
        
    } 
   int x;
   int y;

};
struct PointCmp {
    bool operator()(const point& lhs, const point& rhs) const { 
        return (lhs.x < rhs.x || (lhs.x == lhs.x && lhs.y <= rhs.y )); 
    }
};


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

    std::ios::sync_with_stdio(false);
    int n;
    while(cin >> n){
        int x,y;
        vector <point> points;
        set <point,PointCmp> mnozina;
        int total = n*n;
        for(int j = 0; j < n; j++){
            
            cin >> x >> y;
  
            for(size_t i = 0; i < points.size(); i ++){
                if((abs(points[i].x - x) == abs(points[i].y - y)) && !(x == points[i].x && y == points[i].y)){
                    mnozina.insert(point(x,y));
                    mnozina.insert(point(points[i].x,points[i].y));
                }
            }
            points.push_back(point(x,y));
        }
        /*for(set<point, PointCmp>::iterator it = mnozina.begin(); it != mnozina.end(); it++)
            cout << (*it).x << "," << (*it).y << endl;
        */
        cout << (double)mnozina.size()/(double)total << endl;
        mnozina.clear();
    }
    return 0;
}