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>9&&b>9){
    System.out.println(-1);
    }else{
    System.out.println(a*b);
    }
  }
}