You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
5-(4-chloro-2-fluorophenyl)-7-[3-(difluoromethyl)pyrrolidin-1-yl]-2,3-dimethylpyrido[4,3-d]pyrimidin-4-one is the IUPAC name of a molecule. Please give its molecular formula.
user
C20H18ClF3N4O
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