You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
For the molecule named 1-[2-[1-(3-methoxyphenyl)-2-phenylselanylethoxy]ethyl]-3-methylimidazol-3-ium;tetrafluoroborate by IUPAC nomenclature, provide its molecular formula.
user
C21H25BF4N2O2Se
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