You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
(3S)-1-(3-fluoro-4-methylphenyl)-N-[(2-methoxyphenyl)methyl]-5-oxopyrrolidine-3-carboxamide is the IUPAC name of a molecule. Please give its molecular formula.
user
C20H21FN2O3
assistant