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 (3S)-1-[(2-chlorophenyl)methyl]-3-methyl-4-[(5-methylpyrazin-2-yl)methyl]piperazine-2,5-dione;formic acid into its molecular formula.
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C19H21ClN4O4
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