You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
3-ethyl-N-[3-(5-methyl-1H-1,2,4-triazol-3-yl)phenyl]-4-oxophthalazine-1-carboxamide is the IUPAC name of a molecule. Please give its molecular formula.
user
C20H18N6O2
assistant