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);
		}
		sc.close();
	}
}