import sys
from math import factorial as fact


def add(dic, key):
	if key in dic.keys():
		dic[key] += 1
	else:
		dic[key] = 1

def comb(n, k):
	return fact(n)/(fact(k)*fact(n - k))

while True:
	try:	
		line = sys.stdin.readline()
		if not line:
			break
		N = int(line.rstrip())
		lines = dict()
		for _ in range(N):
			x, y = sys.stdin.readline().rstrip().split()
			x, y = int(x), int(y)
			add(lines, (-1, x + y))
			add(lines, ( 1, y - x))
		s = 0
		for k in lines.values():
			s += (k)*(k - 1)

		print(s/(N*N))		
		
	except EOFError:
		break

exit()