You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
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For the molecule named 3-(4-bromophenyl)sulfanyl-1-[4-(5-nitropyridin-2-yl)piperazin-1-yl]propan-1-one by IUPAC nomenclature, provide its molecular formula.
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C18H19BrN4O3S
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