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 a=0;
      
      for(int i=1;i<=M;i++){
        a +=sc.nextInt();
      }
       if(N-a >=0){
       System.out.println(N-a);
       }
        else if(N-a <0)  {
        System.out.println("-1");
        }
    }
}
/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