It’s a unidirectional ring: you can enter but get out.
Adjoining node is switch for each other.
In each pair of notes, only one node could open.
Pictured above isn't the only one way to enter but the easiest.
Green: Opened Notes(passable); Red: Closed Notes(impassable) ; Yellow: Raw Notes
A pair of Raw Notes(2*yellow)=>actived=>One Green Nodes + One Red Nodes