You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
1-(4-chlorophenyl)-N-[4-(dimethylamino)-2-methylphenyl]-6-methyl-4-oxopyridazine-3-carboxamide The above is the IUPAC name of a molecule. Write its molecular formula.
user
C21H21ClN4O2
assistant