You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
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Under IUPAC nomenclature, a molecule named (7S,9aR)-7-ethyl-6,9-dioxo-N-phenyl-1,3,4,7,8,9a-hexahydropyrazino[1,2-a]pyrazine-2-carboxamide is given. What is its corresponding molecular formula?
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C16H20N4O3
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