import java.util.Scanner;
class Main {
  public static void main(String args[]){
    Scanner sc = new Scanner(System.in);
    int n , m;
    n = sc.nextInt();
    m = sc.nextInt();
    int[] A = new int[m];
    int i = 0;
    for(i=0 ; i < m ; i++){
      A[i] = sc.nextInt();
    }
    for(i = 0; i < m; i++){
      n = n - A[i];
    }
    if(n < 0){
      System.out.println(-1);
    }else{
      System.out.println(n);
    }
  }
}

/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