You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
(4bR,8aR,10aS)-4b,8,8-trimethyl-10a-prop-2-enyl-6,7,8a,9-tetrahydro-5H-phenanthrene-3,10-dione The above is the IUPAC name of a molecule. Write its molecular formula.
user
C20H26O2
assistant