You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
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Translate the chemical IUPAC name 2-[[2-(4-chlorophenyl)pyrimidin-5-yl]methyl]-3,4,6,7,8,8a-hexahydro-1H-pyrrolo[1,2-a]pyrazine into its molecular formula.
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C18H21ClN4
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