import java.util.Scanner;

class A {
  public static void main(String[] args) {
    Scanner stdIn = new Scanner(System.in);

    int a = stdIn.nextInt();
    int b = stdIn.nextInt();
    if (a <= 9 && b <=9){
      System.out.println(a*b);
    }else{
      System.out.println(-1);
    }

  }
}
