You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
(2Z)-2-(10,13-dimethyl-3,11-dioxo-1,2,4,7,8,9,12,14,15,16-decahydrocyclopenta[a]phenanthren-17-ylidene)acetonitrile The above is the IUPAC name of a molecule. Write its molecular formula.
user
C21H25NO2
assistant