You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
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Convert the IUPAC name of a molecule 6-hydroxy-8a-methyl-3-methylidenespiro[3a,4,4a,6,7,8,9,9a-octahydrobenzo[f][1]benzofuran-5,2'-oxirane]-2-one into molecular formula.
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C15H20O4
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