You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
(1R,3aS,6aR)-3,6a-dimethyl-1,2-diphenyl-3a,4,5,6-tetrahydrocyclopenta[b]phosphole 1-oxide The above is the IUPAC name of a molecule. Write its molecular formula.
user
C21H23OP
assistant