import java.util.*;
public class Main{
  public static void main(String[]args){
  Scanner sc=new Scanner(System.in);
  int d=sc.nextInt();
  int as=sc.nextInt();
  int ar[]=new int[10000];
  int s=0;
  
  for(int i=0;i<as;i++){
  ar[i]=sc.nextInt();
  
  }
  for(int i=0;i<ar.length;i++){
  s=s+ar[i];
  }
  int f=d-s;
  if(f>=0){

  System.out.println(f);
  }
  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