You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
(3R,11bS)-9-(3-fluoropropoxy)-10-methoxy-2-methyl-3-(2-methylpropyl)-1,3,4,6,7,11b-hexahydropyrazino[2,1-a]isoquinoline The above is the IUPAC name of a molecule. Write its molecular formula.
user
C21H33FN2O2
assistant