You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
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For the molecule named 1-[(9aR)-1'-(2-phenoxyacetyl)spiro[1,3,4,7,9,9a-hexahydropyrazino[2,1-c][1,4]oxazine-6,3'-azetidine]-8-yl]-2-methylpropan-1-one by IUPAC nomenclature, provide its molecular formula.
user
C21H29N3O4
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