1                                                                                       
                                                                                                                                                  
                                                                                                                                                  
                                                                                                                                                  
                                                                                                                                                 3
                                                                                                          no                                      
                                exists?                                                                                                           
                                                                                                                                                  
                                                                    yes                                                                           
                                                                                                                                                  
                                                                                                                                                  
                                                          2                                                                                      4