import java.util.*;

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

    int N = sc.nextInt();
    int M = sc.nextInt();
    sc.nextLine();

    int[] list = new int[M];

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

    for(int i : list) {
      N -= i;
    }

    if(N <= -1) {
      N = -1;
    }

    System.out.println(N);
    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