import java.util.Scanner;
public class Main {
    public static void main(String[] args) {
        Scanner in=new Scanner(System.in);
        int num=in.nextInt();
        int hw=in.nextInt();
        int sum=0;
        for (int i = 0; i <hw; i++) {
            int x=in.nextInt();
            sum=sum+x;
        }
        int ans=num-sum;
        if(ans<0){
            System.out.println(-1);
        }else{
            System.out.println(ans);
        }
    }
}
/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