You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
N-[4-(4-ethylphenyl)-1,3-thiazol-2-yl]-3-methyl-2-(2-oxopyrrolidin-1-yl)butanamide is the IUPAC name of a molecule. Please give its molecular formula.
user
C20H25N3O2S
assistant