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Under IUPAC nomenclature, a molecule named 3-[(3aR,4S,6aS)-4-phenyl-2,3,3a,4,6,6a-hexahydro-1H-pyrrolo[3,4-c]pyrrole-5-carbonyl]pyrido[1,2-a]pyrimidin-4-one is given. What is its corresponding molecular formula?
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C21H20N4O2
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