import java.util.Scanner;

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

		if (A <= 9 && B <= 9) {
			System.out.println(A * B);
		} else {
			System.out.println(-1);
		}
	}
}