Push Inside!

Spoiler: hint

There is a hidden piece in the first tunnel. To use it, you must first activate the mechanism.
R and B are used respectively. And finally, using a slider and running on the ceiling of the tunnel, B pushes the secret piece.
however,It is possible, that the pushing operation is necessary to repeat two or three times. For this reason, some of sliders used to prevent the restart.
good luck
(Especially you gmacpro)