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