You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
1-(3-chlorophenyl)-4-(7-methoxy-1-methyl-3,4-dihydro-1H-isoquinolin-2-yl)butane-1,4-dione is the IUPAC name of a molecule. Please give its molecular formula.
user
C21H22ClNO3
assistant