You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
(3aS,3bR,9aR,9bS,11aS)-9a,11a-dimethyl-2,3,3a,3b,4,5,7,8,9,9b,10,11-dodecahydronaphtho[2,1-e][1]benzofuran is the IUPAC name of a molecule. Please give its molecular formula.
user
C18H28O
assistant