You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
(7-chloro-4-oxopyrido[1,2-a]pyrimidin-2-yl)methyl 3-(4-methylphenyl)thiophene-2-carboxylate The above is the IUPAC name of a molecule. Write its molecular formula.
user
C21H15ClN2O3S
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