import java.util.Scanner;
public class Main
{
    public static void main(String Args[])
    {
        Scanner sc=new Scanner (System.in);
        int n=sc.nextInt();
        int m=sc.nextInt();
        int[] x=new int[m];
        for(int a=0;a<m;a++)
        {
            x[a]=sc.nextInt();
            n=n-x[a];
        }
        
        if (n<0)
        System.out.println("-1");
        if(n>=0)
        System.out.println(n);
    }
}
/tmp/hf-datasets-cache/medium/datasets/42704881117311-config-parquet-and-info-qiankunmu-Project_CodeNet-896c9d36/hub/datasets--qiankunmu--Project_CodeNet_Python800_and_Java250/snapshots/c899fde44a77ba1cc14973ed3de064e4ff3aadc7/Project_CodeNet_Java250.tar.gz