You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
methyl 4-[(3aR,4S,9bR)-8-bromo-3a,4,5,9b-tetrahydro-3H-cyclopenta[c]quinolin-4-yl]benzoate is the IUPAC name of a molecule. Please give its molecular formula.
user
C20H18BrNO2
assistant