You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
(4aS,4bS,8aR,8bS)-2-methoxy-8a-(4-methoxyphenyl)-4b,5,6,7,8,8b-hexahydro-4aH-biphenylene-1,4-dione is the IUPAC name of a molecule. Please give its molecular formula.
user
C20H22O4
assistant