You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
(7R,8S,9S,13R,14S)-13-(ethylperoxymethyl)-7-methyl-7,8,9,12,14,15,16,17-octahydro-6H-cyclopenta[a]phenanthren-11-one is the IUPAC name of a molecule. Please give its molecular formula.
user
C21H28O3
assistant