You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
4-[4-[4-hydroxy-5-[(E)-indol-3-ylidenemethyl]-2-sulfanylidene-1,3-thiazol-3-yl]butanoylamino]butanoic acid The above is the IUPAC name of a molecule. Write its molecular formula.
user
C20H21N3O4S2
assistant