#coding: UTF-8

import sys
import re
import itertools
from math import log
from collections import deque

### defs ###


### main ###
N = int(sys.stdin.readline())
AB = []
for _ in range(N):
	a,b=map(int,sys.stdin.readline().split())
	AB.append((a,b))

if (len(list(filter(lambda x:(x[0]+4*x[1])%2==0,AB))) != 0 and len(list(filter(lambda x:(x[0]+4*x[1])%2==1,AB))) != 0):
	print(-1)
else:
	mx = max(map(lambda x:x[0]+4*x[1], AB))
	ans = sum(map(lambda x:(mx - x[0]-4*x[1])//2, AB))
	print(ans)