import java.util.Scanner;

public class NineNine {
	
		public static void main(String[] args) {
			
			Scanner sc =new Scanner(System.in);
			
			
			int a,b;
			a=sc.nextInt();
			b=sc.nextInt();
			
			if(a>9||b>9) {
				System.out.println(-1);
				
			}
			else {
				System.out.println(a*b);
			}
			
				
		}

}
