import java.util.*;

class Main{
    public static void main(String args[]){
         Scanner S =  new Scanner(System.in);
         int N = Integer.parseInt(S.next());
         int M = Integer.parseInt(S.next());
         int total = 0;
         for(int i=0;i<M;i++){
             total += Integer.parseInt(S.next());
         }

         if(N >= total){
             System.out.println(N-total);
         } 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