import java.util.*;
import java.lang.*;

public class Main{

  public static void main(String args[]){
  
    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();
    int m = sc.nextInt();
int sum=0;
    while(m--!=0){
int a = sc.nextInt();
sum+=a;
}
if(sum>n) System.out.println(-1);
else {
System.out.println(n-sum);
}
  }

}
/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