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Name the molecular formula for the molecule established as ethyl 2-[(4S)-7-[(E)-but-2-enyl]-3,4,9-trimethyl-6,8-dioxo-4H-purino[8,7-c][1,2,4]triazin-1-yl]acetate using the IUPAC nomenclature.
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C18H24N6O4
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