# -*- coding:utf-8 -*-
import sys

if __name__ == "__main__":
	n = input()
	l = []
	for i in xrange(n):
		a,b = map(int,raw_input().split())
		l.append(a + 4 * b)
	max_l = max(l)
	ans = 0
	for i in l:
		sub = max_l - i
		if sub % 2 == 1:
			print -1
			sys.exit()
		else:
			ans += sub /2
	print ans