You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
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For the molecule named [(3aR,5R,5aR,8aR,9S,9aR)-5,8a-dimethyl-1-methylidene-2,8-dioxo-3a,4,5,5a,9,9a-hexahydroazuleno[6,5-b]furan-9-yl] 2-methylbutanoate by IUPAC nomenclature, provide its molecular formula.
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C20H26O5
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