You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
1-[(2-chloro-1,3-thiazol-5-yl)methyl]-2-hydroxy-3-(2-methoxyphenyl)-9-methylpyrido[1,2-a]pyrimidin-1-ium-4-one is the IUPAC name of a molecule. Please give its molecular formula.
user
C20H17ClN3O3S+
assistant