You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
(3S,3aS,5aS)-3,5a,9-trimethyl-1,3,3a,4,5,9b-hexahydrobenzo[g]indole-2,8-dione The above is the IUPAC name of a molecule. Write its molecular formula.
user
C15H19NO2
assistant