You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
3-[2,6-dihydroxy-3-(2-oxo-5-sulfoindol-3-yl)furo[2,3-f][1]benzofuran-7-yl]-2-oxoindole-5-sulfonic acid is the IUPAC name of a molecule. Please give its molecular formula.
user
C26H12N2O12S2
assistant