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-[(4-fluorophenyl)methyl]-9-hydroxy-7-(3-methylbutanoyl)pyrido[1,2-a]pyrazine-1,8-dione;hydrochloride into its molecular formula.
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C20H20ClFN2O4
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