You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
For the molecule named 3-(4-acetylpiperazin-1-yl)-1-butyl-5,6,7,8-tetrahydroisoquinolin-2-ium-4-carbonitrile by IUPAC nomenclature, provide its molecular formula.
user
C20H29N4O+
assistant