You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
1-[2-(1-acetylpiperidin-2-yl)-7,8-dihydro-5H-pyrido[4,3-d]pyrimidin-6-yl]-2-(2-ethylimidazol-1-yl)ethanone The above is the IUPAC name of a molecule. Write its molecular formula.
user
C21H28N6O2
assistant