package yukicoder178;
import java.util.Scanner;
public class Main {
	public static void main(String[] args){
		Scanner sc=new Scanner(System.in);
		int n=sc.nextInt();
		int[] a=new int[n];//space
		int[] b=new int[n];//tab
		int[] width=new int[n];
		int min=999999;
		for(int i=0;i<n;i++){
			a[i]=sc.nextInt();
			b[i]=sc.nextInt();
			width[i]=a[i]+4*b[i];
			min=Math.min(width[i], min);
		}		
		int ans=0;
		for(int i=0;i<n;i++){
			width[i]-=min;
			if(width[i]%2==1){
				System.out.println(-1);
				return;
			}else{
				ans+=width[i]/2;
			}
		}
		System.out.println(ans);
	}
}