21

          
                      20    

          
                      19    ȷ

           
                     18

               
         17
                                

            
       
                                                                              16

                
                                        15                                           14           13          12

                       
                     11                                          10                  9                    8

             
                    7                                                                  6

                       
                5                          4                     3               2

  
                         1