import java.util.*;
public class Main {
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
      int N=sc.nextInt();
      int M=sc.nextInt();
      System.out.println();
      int A[]=new int[M];
      int total=0;
      for(int i=0;i<M;i++){
         A[i]=sc.nextInt();
         total+=A[i];
      }
      if(N-total>=0){
         System.out.println(N-total);
      }else{
         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