You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
For the molecule named 1-(3-chloro-4-methoxyphenyl)-N-(2-methoxy-5-methylphenyl)-5-oxopyrrolidine-3-carboxamide by IUPAC nomenclature, provide its molecular formula.
user
C20H21ClN2O4
assistant