You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
3-ethoxycarbonyl-1,2-dimethyl-4-[(4-methylpiperidin-1-ium-1-yl)methyl]indol-5-olate is the IUPAC name of a molecule. Please give its molecular formula.
user
C20H28N2O3
assistant