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 >= 10 || B >= 10)
      System.out.println("-1");
    else
      System.out.println(A*B);
  }
}
