#coding:utf-8

n=input()
a=[raw_input().split() for i in range(n)]

s=set()
for i in a:
	s.add(int(i[1])-int(i[0]))

s_list=list(s)
	
if len(s_list) == 1 and 0 < s_list[0]:
	print s_list[0]
else:
	print -1