You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
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For the molecule named (1R,4E,7aS)-4-(bromomethylidene)-1-[(1R)-1-hydroxyethyl]-7a-methyl-2,3,3a,5,6,7-hexahydroinden-1-ol by IUPAC nomenclature, provide its molecular formula.
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C13H21BrO2
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