You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
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For the molecule named 4,7,8-trimethyl-6-(2-methylphenyl)-2-(2-oxopropyl)purino[7,8-a]imidazole-1,3-dione by IUPAC nomenclature, provide its molecular formula.
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C20H21N5O3
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