     
       
       19     20                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            
                                                      
               4  5             4  5       40  50                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   10                                                                                                                                                                                  
                                        