#include <stdio.h>
#include <map>

// double prob_line(int count, int total)
// {
//   double ret = 0;
//   while(count--) {
//     double tmp = 0;
//     tmp += 2*count;
//     tmp /= total*total;
//     ret += tmp;
//   }
//   return ret;
// }



int factorial(int N){
  int result = 1;
  while (N > 0){
    result *= N;
    N--;
  }
  return result;
}

double combinate(int n, int k){
  int bigger = k > n -k?  k : n-k;
  int smaller = k < n -k?  k : n-k;

  int result = 1;
  while( n > bigger ) {
    result *= n;
    n--;
  }
  return ((double)result)/factorial(smaller);
}

double get_prob(std::map<int , int>& map, int total)
{
  std::map<int, int>::iterator it = map.begin();
  double ret = 0;
  while(it != map.end()) {
    ret += it->second*it->second - it->second;
    it++;
  }
  return (ret);
}


int main()
{

  int N;
  int total;
  while(scanf("%d", &N) != EOF) {
    std::map<int, int> map_right, map_left;
    total = N;
    double res = 0;

    while(N--) {
      int x, y;

      scanf("%d %d" , &x, &y);

      if ( map_right.end() == map_right.find(y-x))
	map_right[y-x] = 0;
      map_right[y-x]++;

      if ( map_left.end() == map_left.find(y+x))
	map_left[x+y] = 0;
      map_left[y+x]++;

    }

    res += get_prob(map_right, total);
    res += get_prob(map_left, total);
    printf("%.6g\n", res/(total*total));
  }
  return 0;
}