#!/usr/bin/python
# -*- coding: utf-8 -*-

N = int(raw_input())
arr = [0 for i in range(N) ]
arr2 = []
for i in range(N):
	A,B= map(int, raw_input().split())
	arr[i] = A + 4 * B

arr.sort()
arr.reverse()
for i in range(1,N):
	arr2.append(arr[0] - arr[i])
ans = 0
for i in arr2:
	if i % 2 != 0:
		print "-1"
		exit()
	ans += i / 2
print ans