import java.util.*;

class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);

    int days = sc.nextInt();
    int works = sc.nextInt();

    int[] workList = new int[works];

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

    for (int i = 0; i < workList.length; i++) {
      days -= workList[i];
    }

    if (days < 0) {
      System.out.println("-1");
    } else {
      System.out.println(days);
    }

    sc.close();

  }

}
/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