You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
For the molecule named methyl 2-[4-[4-(6-amino-2-butoxy-8-oxo-7H-purin-9-yl)butyl]piperazin-1-yl]acetate by IUPAC nomenclature, provide its molecular formula.
user
C20H33N7O4
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