You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
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Convert the IUPAC name of a molecule N-[3-(3,4,6,7,9,9a-hexahydro-1H-pyrazino[2,1-c][1,4]oxazine-8-carbonyl)thiophen-2-yl]-4-ethylbenzamide into molecular formula.
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C21H25N3O3S
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