import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		String l = sc.nextLine();
		String[] arr1 = l.split(" ");
		l = sc.nextLine();
		String[] arr2 = l.split(" ");

		Integer dSum = 0;
		for(String s : arr2) {
			dSum += Integer.valueOf(s);

		}
		Integer res = Integer.valueOf(arr1[0]) - dSum;
		System.out.println(res >= 0 ? res : -1);
		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