You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
1-[1-(5-chloropyrazin-2-yl)piperidin-4-yl]-3-(2,5-difluoro-4-methylsulfonylanilino)pyrrolidin-2-one is the IUPAC name of a molecule. Please give its molecular formula.
user
C20H22ClF2N5O3S
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