You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
Convert the IUPAC name of a molecule (1R,4S)-1-[(2S)-butan-2-yl]-1,8-dimethoxy-4-propan-2-yl-2,4-dihydropyrazino[2,1-b]quinazoline-3,6-dione into molecular formula.
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C20H27N3O4
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