You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
(8R,13S,14S,17R)-17-hydroxy-13-methyl-17-prop-2-enyl-1,2,6,7,8,14,15,16-octahydrocyclopenta[a]phenanthren-3-one The above is the IUPAC name of a molecule. Write its molecular formula.
user
C21H26O2
assistant