You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
(8R,8aS)-8-butyl-8-methyl-5,6,7,8a-tetrahydro-[1,3]thiazolo[3,2-a]pyrazin-3-one is the IUPAC name of a molecule. Please give its molecular formula.
user
C11H20N2OS
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