#coding: utf-8
##yuki_185
import sys

n=int(raw_input())

for i in xrange(n):
 s=map(int,raw_input().split())
 x=s[0]
 y=s[1]
 if i==0:
  if y-x>=0:
   temp=y-x
  else:
   print -1
   sys.exit()
 else:
  if y-x>=0 and temp==y-x:
   continue
  else:
   print -1
   sys.exit()

res=temp
print res