You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
(5R)-3-[2-[(2S)-2-methyl-2,3-dihydroindol-1-yl]-2-oxoethyl]-5-(2-methylpropyl)imidazolidine-2,4-dione is the IUPAC name of a molecule. Please give its molecular formula.
user
C18H23N3O3
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