You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
ethyl 2-[9-(3,4-dimethylphenyl)-1-methyl-2,4-dioxo-6,7,8,10-tetrahydropurino[7,8-a]pyrimidin-5-ium-3-yl]acetate is the IUPAC name of a molecule. Please give its molecular formula.
user
C21H26N5O4+
assistant