You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
(6aR,10aR)-4-piperazin-1-yl-6a,7,8,9,10,10a-hexahydro-6H-indolo[4,3-fg]quinoline is the IUPAC name of a molecule. Please give its molecular formula.
user
C18H24N4
assistant