You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
3-ethyl-1-methyl-6-[(2S)-1-thiophen-3-ylpropan-2-yl]pyrazolo[4,3-d]pyrimidin-7-one is the IUPAC name of a molecule. Please give its molecular formula.
user
C15H18N4OS
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