You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
N-(4-acetylphenyl)-2-(6,7-dimethyl-3-oxo-2,4-dihydro-1H-quinoxalin-2-yl)acetamide is the IUPAC name of a molecule. Please give its molecular formula.
user
C20H21N3O3
assistant