You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
(3R,7aS)-N-(1-benzylpyrazol-3-yl)-7a-methyl-5-oxo-2,3,6,7-tetrahydropyrrolo[2,1-b][1,3]thiazole-3-carboxamide is the IUPAC name of a molecule. Please give its molecular formula.
user
C18H20N4O2S
assistant