import java.util.Scanner;

public class Main {
	public static void main(String[] args) throws Exception {
		Scanner sc = new Scanner(System.in);
		int a = sc.nextInt();
		int b = sc.nextInt();
		sc.close();

		if (a < 10 && b < 10) {
			System.out.println(a * b);
		} else {
			System.out.println(-1);
		}
	}
}
