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

if __name__ == "__main__":
	n = input()
	for i in xrange(n):
		a,b = map(int,raw_input().split())
		if b - a < 0:
			print -1
			sys.exit()
		if i == 0:
			c = b-a
		else:
			if b-a != c:
				print -1
				sys.exit()
	print c