#include <iostream>

using namespace std;

struct Point
{
    int X;
    int Y;
    Point()
    {
        X = Y = 0;
    }
    Point(int x, int y)
    {
        X = x;
        Y = y;
    }
};

int main()
{
    std::ios_base::sync_with_stdio(false);
    Point* points;
    int pocet;

    while(true)
    {
        cin >> pocet;

        if(cin.eof())
            break;

        points = new Point[pocet];
        for(int i = 0; i < pocet; i++)
        {
            cin >> points[i].X;
            cin >> points[i].Y;
        }

        int celkem = pocet * pocet;
        int jeVyhra = 0;

        for(int i = 0; i < pocet; i++)
        {
            for(int j = i + 1; j < pocet; j++)
            {
                if(abs(points[i].X - points[j].X) == abs(points[i].Y - points[j].Y))
                {
                    jeVyhra+=2;
                }
            }
        }

        cout << (float)jeVyhra / celkem << endl;

        delete[] points;
    }

    return 0;
}