You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
Translate the chemical IUPAC name 3-[2-[3-(2,3-dihydroindol-1-yl)-3-oxopropyl]sulfanylbenzimidazol-1-yl]propanenitrile into its molecular formula.
user
C21H20N4OS
assistant