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