You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
2-(1,3-dimethyl-2,4-dioxo-7-propan-2-ylpyrimido[4,5-d]pyrimidin-5-yl)sulfanylacetic acid is the IUPAC name of a molecule. Please give its molecular formula.
user
C13H16N4O4S
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