#coding: utf-8
##yuki_178
import sys

st_max=0
st_min=sys.maxint

n=int(raw_input())
for i in xrange(n):
 a,b=map(int,raw_input().split())
 st=a+b*4
 if st>st_max:
  st_max=st
 if st<st_min:
  st_min=st
  
#print st_max
#print st_min
res=st_max-st_min
if res%2==1:
 print -1
else:
 print res/2