You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
(3S)-3-[[2-[(4-ethyl-2,3-dioxopiperazine-1-carbonyl)amino]-2-phenylacetyl]amino]-2-oxoazetidine-1-sulfonic acid The above is the IUPAC name of a molecule. Write its molecular formula.
user
C18H21N5O8S
assistant