package main

import . "fmt"

func main() {
	var n,a,b int64
	Scan(&n,&a,&b)
	max:=a+4*b
	sum:=max
	rem:=max%2
	for i:=int64(1);i<n;i++ {
		Scan(&a,&b)
		x:=a+4*b
		if rem!=x%2 {
			Println(-1)
			return
		} 
		if x>max {
			max=x
		}
		sum+=x
	}
	ans:=(max*n-sum)/2
	Println(ans)
}