You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
3-[[4-[(Z)-2-carboxy-2-[(5-ethyl-1,3,4-oxadiazol-2-yl)sulfanyl]ethenyl]phenoxy]methyl]benzoic acid The above is the IUPAC name of a molecule. Write its molecular formula.
user
C21H18N2O6S
assistant