You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
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2-(3-bromo-4-chlorophenyl)-3-ethyl-6,7,8,8a-tetrahydro-3H-pyrrolo[1,2-a]pyrazine-1,4-dione is the IUPAC name of a molecule. Please give its molecular formula.
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C15H16BrClN2O2
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