#coding: utf-8
##yuki_178
import sys

st_max=0
ws=[]
wsc=0

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
 ws.append(st)
 
for i in ws:
 if (st_max-i)%2==1:
  print -1
  sys.exit()
 else:
  wsc+=(st_max-i)/2
  
print wsc