def north(n):
  a = {}
  b = {}
  sum = 0

  for i in range(n):
    coord = [int(x) for x in input().split(' ')]
    a[coord[1] - coord[0]] = a.get(coord[1] - coord[0], 0) + 1
    b[coord[1] + coord[0]] = b.get(coord[1] + coord[0], 0) + 1
 
  for v in a.values():
    sum += (v*(v-1))
  
  for v in b.values():
    sum += (v*(v-1))

  n = n*n

  return sum/n


def main():

  while (True):
    try:
      x = input()
      if not x:
        break
      n = int(x)
      print(north(n))

    except:
      return


main()