You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
3-ethyl-8-fluoro-7-[2-[2-fluoro-4-(hydroxymethyl)phenyl]ethyl]-3,4-dihydroisochromen-1-one is the IUPAC name of a molecule. Please give its molecular formula.
user
C20H20F2O3
assistant