You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
3-(7,7-dimethyl-4-oxo-1,3,6,11b-tetrahydropyrazino[2,1-a]isoquinoline-2-carbonyl)benzonitrile is the IUPAC name of a molecule. Please give its molecular formula.
user
C22H21N3O2
assistant