You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
For the molecule named [(3S,8S,9S,10R,13S,14S)-10,13-dimethyl-7,17-dioxo-2,3,4,8,9,11,12,14,15,16-decahydro-1H-cyclopenta[a]phenanthren-3-yl] acetate by IUPAC nomenclature, provide its molecular formula.
user
C21H28O4
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