You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
5-(3,4-difluorophenyl)-5-methyl-3-[2-(4-methylpiperidin-1-yl)-2-oxoethyl]imidazolidine-2,4-dione is the IUPAC name of a molecule. Please give its molecular formula.
user
C18H21F2N3O3
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