You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
7-[(5-methyl-1H-indol-3-yl)methyl]-2-methylsulfonyl-3,5,6,8-tetrahydropyrido[3,4-d]pyrimidin-4-one is the IUPAC name of a molecule. Please give its molecular formula.
user
C18H20N4O3S
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