import java.util.Scanner;

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