You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
Translate the chemical IUPAC name N-[[(E)-2-methyl-3-phenylprop-2-enylidene]amino]-3-(3-oxo-4H-quinoxalin-2-yl)propanamide into its molecular formula.
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C21H20N4O2
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