You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
(3aS,4S,9bS)-6-methoxy-4-(4-methylphenyl)-9-nitro-3a,4,5,9b-tetrahydro-3H-cyclopenta[c]quinoline is the IUPAC name of a molecule. Please give its molecular formula.
user
C20H20N2O3
assistant