20
30
------------------------------
------------------------------
------------------------------
------------------------------
-------------------X----------
------------------XX----------
--------XX---XXXXXXXX---------
---------XX----XXXXXXXX-------
----------XXXXXXXXXXXXXXXX----
----------XXXXXXXXX-----------
----------XXXXX--XX-----------
-----------XXX----XX----------
-------------X-----XX---------
-----------------XXX----------
-----------------XXX----------
-----------------X------------
------------------------------
------------------------------
--------a---------------------
------------------------------
------------------------------