You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
1-[(1S)-1-(4-chloro-3-methoxyphenyl)-6,7-dimethoxy-3,4-dihydro-1H-isoquinolin-2-yl]ethanone is the IUPAC name of a molecule. Please give its molecular formula.
user
C20H22ClNO4
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