You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
5-[3-methyl-4-(2-phenylethyl)piperazin-1-yl]sulfonyl-1,3-dihydrobenzimidazol-2-one is the IUPAC name of a molecule. Please give its molecular formula.
user
C20H24N4O3S
assistant