You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
For the molecule named (4R,7aS,12bS)-7-methylidene-3-prop-2-enyl-2,4,5,6,7a,13-hexahydro-1H-4,12-methanobenzofuro[3,2-e]isoquinoline-4a,9-diol by IUPAC nomenclature, provide its molecular formula.
user
C20H23NO3
assistant