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 8-(6-methyl-3-nitropyridin-2-yl)-3-[2-(6-methylpyridin-2-yl)ethynyl]-1-oxa-2,8-diazaspiro[4.5]dec-2-ene into molecular formula.
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C21H21N5O3
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