import java.util.*;

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