You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
For the molecule named 2-[(1R,2S)-6,7-dimethoxy-2-methyl-1-phenyl-3,4-dihydro-1H-isoquinolin-2-ium-2-yl]acetamide by IUPAC nomenclature, provide its molecular formula.
user
C20H25N2O3+
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