import java.util.Scanner;

public class Main {
  public static void main(String[] main) {
    Scanner input = new Scanner(System.in);
    String[] ss = input.nextLine().split(" ");
    int n = Integer.parseInt(ss[0]);
    int m = Integer.parseInt(ss[1]);
    ss = input.nextLine().split(" ");
    int total = 0;
    for (int i = 0; i < m; i++) {
      total += Integer.parseInt(ss[i]);
    }
    if (n < total) {
      System.out.println("-1");
    } else {
      System.out.println(n - total);
    }
  }
}
/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