import java.util.Scanner;
public class Main{
  public static void main(String[] args){
    Scanner sc = new Scanner(System.in);
    int c = sc.nextInt();
    int a,b,d;
    a = sc.nextInt();
    int x[] = new int [a];
    d = 0;
    for(b =0;b < a;b++){
      x[b] = sc.nextInt();
      d = d + x[b];
    }
    int e = c-d;
    if(e >= 0){
      System.out.println(e);
    }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