You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
Please write the molecular formula of the molecule 1-butyl-4-(4-ethylphenyl)-6,6-dioxo-4a,5,7,7a-tetrahydro-2H-thieno[3,4-b]pyrazin-3-one .
user
C18H26N2O3S
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