You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
(2R,3R)-1-methyl-N-[(2-methylphenyl)methyl]-5-oxo-2-thiophen-2-ylpyrrolidine-3-carboxamide is the IUPAC name of a molecule. Please give its molecular formula.
user
C18H20N2O2S
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