You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
Convert the IUPAC name of a molecule (4-butan-2-ylphenyl) 1a,2,2a,3,4,5,6,6a,7,7a-decahydronaphtho[2,3-b]thiirene-4-carboxylate into molecular formula.
user
C21H28O2S
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