You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
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For the molecule named (8aR)-7-(4-methyl-2-nitrophenyl)sulfonyl-1,2,5,6,8,8a-hexahydroimidazo[1,5-a]pyrazin-3-one by IUPAC nomenclature, provide its molecular formula.
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C13H16N4O5S
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