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What is the molecular formula of the compound with this IUPAC name (3aR,6aR)-5-acetyl-2-[(4-oxopyrido[1,2-a]pyrimidin-2-yl)methyl]-3,4,6,6a-tetrahydro-1H-pyrrolo[3,4-c]pyrrole-3a-carboxylic acid ?
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C18H20N4O4
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