You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
8-(4-methoxy-2-methylphenyl)-1,2,3,4,4a,6-hexahydropyrazino[1,2-a]quinoxalin-5-one The above is the IUPAC name of a molecule. Write its molecular formula.
user
C19H21N3O2
assistant