You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
(4R,7aR,12bS)-9-fluoro-7-methoxy-3-methyl-2,4,7a,13-tetrahydro-1H-4,12-methanobenzofuro[3,2-e]isoquinoline is the IUPAC name of a molecule. Please give its molecular formula.
user
C18H18FNO2
assistant