import java.util.*;

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(A * B);
    }else{
      System.out.println(-1);
    }
  }
}
