import java.util.Scanner;

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

   if(sum<=N){
     System.out.println(N-sum);
   }
   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