import sys

lines = sys.stdin.read().split('\n')
i=0
while i < len(lines):
	if not lines[i]: break
	count = int(lines[i])
	i += 1
	pravo = {}
	lavo = {}

	for j in range(count):
		[x, y] = [int(x) for x in lines[i + j].split(' ')]

		if x+y not in lavo: lavo[x+y] = 0
		lavo[x+y] += 1
		
		if x-y not in pravo: pravo[x-y] = 0
		pravo[x-y] += 1

	i += count

	all = count * count
	win = 0

	for v in pravo.values():
		win += v * (v-1)
	for v in lavo.values():
		win += v * (v-1)

	print('%.7f' % (win/all))
	#print('w:%d all:%d === %.7f' % (win, all, win/all))

	#//		printf("%.7f\n", win/double(all));
	#printf("w:%lld all:%lld  === %.7f\n", win, all, win/double(all));