You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
For the molecule named N-[2-(1-acetyl-1,4-diazaspiro[5.5]undecan-4-yl)-2-oxoethyl]-2-methoxybenzamide by IUPAC nomenclature, provide its molecular formula.
user
C21H29N3O4
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