import sys

while 1:
  try:
    n = int(input())
  except:
    break
  ppoile = {}
  ppoilex = {}
  for xxx in range(n):
    x, y = input().split()
    x = int(x)
    y = int(y)
    ppoile[x-y] = ppoile.get(x-y, 0) + 1
    ppoilex[-y-x] = ppoilex.get(-y-x, 0) + 1
  sumark = 0
  for k, v in ppoile.items():
    sumark += v * (v-1)
  for k, v in ppoilex.items():
    sumark += v * (v-1)
  print(sumark/(n*n))