2-3
                                          |
                                        4-1    11-12
                                        |       |
                                        5    9-10
                                        |    |  |
                                       *6*-7-8 13-14
                                             |     |
                                            16    15
                                             |
                                        24  17
                                         |   |
                                        23--18-*19*-20
                                                    |
                                                   21-22