You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
7-hydroxy-1,1,4a-trimethyl-8-propan-2-yl-3,4,10,10a-tetrahydro-2H-phenanthren-9-one is the IUPAC name of a molecule. Please give its molecular formula.
user
C20H28O2
assistant