You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
(4aR,4bS,7S,8aR)-7-ethyl-7,8-dimethyl-3,4,4a,4b,5,6,8,8a,9,10-decahydrophenanthren-2-one is the IUPAC name of a molecule. Please give its molecular formula.
user
C18H28O
assistant