import java.util.Scanner;

public static void main(String[] args){
  
  Scanner stdIn = new Scanner(System.in);
  
  int a = stdIn.nextInt();
  int b = stdIn.nextInt();
  	if ( a * b > 10){
      System.out.println(a*b);
    }elise{
      System.out.println(-1);
      
    }
}