import java.util.Scanner;

public class Main{
	public static void main(String[] args){
		Scanner sc=new Scanner(System.in);
		while(sc.hasNext()){
			int n=sc.nextInt();
			long[] c=new long[n];
			long max=0;
			for(int i=0;i<n;i++){
				c[i]=sc.nextInt()+sc.nextInt()*4;
				max=Math.max(c[i],max);
			}
			long ans=0;
			for(int i=0;i<n;i++){
				if(ans==-1) break;
				if((max-c[i])%2==1) ans=-1;
				else ans+=(max-c[i])/2;
			}
			System.out.println(ans);
		}
	}
}