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

if __name__ == "__main__":
	n = input()
	nums = []
	for i in xrange(n):
		a,b = map(int,raw_input().split())
		nums.append(b-a)
	nums = set(nums)
	nums = list(nums)
	if len(nums) == 1 and nums[0] >0:
		print  nums[0]
	else:
		print -1