You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
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For the molecule named (13S)-13,15-dimethylspiro[1,2,8,9,10,11,12,14,15,16-decahydrocyclopenta[a]phenanthrene-17,5'-2H-furan]-3-one by IUPAC nomenclature, provide its molecular formula.
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C22H28O2
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