You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
1-[(3aS,11bR)-3-acetyl-1,2,3a,4,5,7-hexahydropyrrolo[2,3-d]carbazol-6-yl]ethanone is the IUPAC name of a molecule. Please give its molecular formula.
user
C18H20N2O2
assistant