You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
4-methoxy-N-(5-methyl-6H-oxadiazolo[4,3-a]phthalazin-4-ium-1-yl)benzamide is the IUPAC name of a molecule. Please give its molecular formula.
user
C18H17N4O3+
assistant