#include <iostream>
#include <map>
#include <cmath>

int main(int argc, char const *argv[])
{
  int n;
  while (std::cin >> n)
  {
    int x, y;
    std::map<int, int> cnts_ne, cnts_nw;
    for (int i = 0; i < n; ++i)
    {
      std::cin >> x >> y;
      cnts_ne[x - y]++;
      cnts_nw[x + y]++;
    }

    int possibs = 0;
    for (const auto &item : cnts_ne)
    {
      if (item.second < 2) continue;
      possibs += item.second * (item.second - 1);
    }
    for (const auto &item : cnts_nw)
    {
      if (item.second < 2) continue;
      possibs += item.second * (item.second - 1);
    }

    double p = double(possibs) / pow(n, 2);
    std::cout << p << '\n';
  }

  return 0;
}