You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
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For the molecule named (5S)-5-(3,4-dimethoxyphenyl)-2-propylsulfanyl-3,5,6,8-tetrahydropyrido[2,3-d]pyrimidine-4,7-dione by IUPAC nomenclature, provide its molecular formula.
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C18H21N3O4S
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