You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
3-benzoyl-9-methyl-2,5,6,10b-tetrahydro-1H-pyrazolo[5,1-a]isoquinoline-1-carbonitrile The above is the IUPAC name of a molecule. Write its molecular formula.
user
C20H19N3O
assistant