You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
[[(6aR,10aR)-7-methyl-5-methylsulfanyl-6,6a,8,9,10,10a-hexahydro-4H-indolo[4,3-fg]quinolin-9-yl]-(dimethylamino)methyl] formate is the IUPAC name of a molecule. Please give its molecular formula.
user
C20H27N3O2S
assistant