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();
      int[] a = new int[M];
      

      for(int i = 0;i < M;i++){
        a[i] = sc.nextInt();
      }

      for(int i = 0;i < a.length;i++){
         N -= a[i];
      }
         if(N >= 0){
            System.out.println(N);
         }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