import java.util.Scanner;

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

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