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);
      return;
    }
    System.out.println(a*b);
  }
}