You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
1-(5-ethyl-1,2-dihydropyridin-3-yl)-8,9-dimethoxy-3-methyl-5,6-dihydroimidazo[5,1-a]isoquinoline The above is the IUPAC name of a molecule. Write its molecular formula.
user
C21H25N3O2
assistant