#coding: utf-8
##yuki_185

import sys

s=[]
n=int(raw_input())
for i in xrange(n):
 s.append(raw_input())

l=[]
for i in range(0,len(s)):
 ltt=map(int,s[i].split())
 lt=ltt[:2]
 l.append(lt)
print l

res=l[0][1]-l[0][0]
if res<0:
 print -1
 sys.exit()

for i in xrange(1,len(l)):
 if len(l[i])==1:
  break
 if l[i][1]-l[i][0]<0 or l[i][1]-l[i][0] != res:
  print -1
  sys.exit()
print res