You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
(4S)-4,4a,7,8-tetramethyl-5-nitroso-7-prop-1-en-2-yl-4,4b,5,6,8,8a,9,10-octahydro-3H-phenanthren-2-one The above is the IUPAC name of a molecule. Write its molecular formula.
user
C21H31NO2
assistant