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 (4-chloro-4a,5,7,8-tetrahydro-2H-pyrido[4,3-d]pyrimidin-6-yl)-(4-phenylphenyl)methanone is given. What is its corresponding molecular formula?
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C20H18ClN3O
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