import java.util.Arrays;
import java.util.Scanner;

public class Main {

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

        final int n = sc.nextInt();
        final int m = sc.nextInt();

        final int[] a = new int[m];
        for (int i = 0; i < m; i++) {
            a[i] = sc.nextInt();
        }

        int sum = Arrays.stream(a).sum();
        System.out.println(sum > n ? -1 : n - sum);
    }
}

/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