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