You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
(13'S)-10',13'-dimethylspiro[1,3-dioxolane-2,17'-7,8,9,11,12,14,15,16-octahydro-6H-cyclopenta[a]phenanthrene]-3'-one The above is the IUPAC name of a molecule. Write its molecular formula.
user
C21H28O3
assistant