import java.util.Scanner;

public class Main {

    private void execute() throws Exception {
        Scanner sc = new Scanner(System.in);
        String ab = sc.nextLine();
        String[] abs = ab.split(" ");
        int a = Integer.parseInt(abs[0]);
        int b = Integer.parseInt(abs[1]);
        if (a >= 10 || b >= 10) {
            System.out.println(-1);
            return;
        }
        System.out.println(a * b);
    }

    public static void main(String[] args) {
        try {
            new Main().execute();
        } catch (Exception ex) {
            ex.printStackTrace();
        }
    }

}

/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