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 <10 && B < 10) {
			System.out.println(A*B);
		}else {
			System.out.println("-1");
		}
		
		sc.close();
	}
}