You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
1-(7-ethyl-1H-indol-3-yl)-2-[[4-(2-methylphenyl)-1,2,4-triazol-3-yl]sulfanyl]ethanone is the IUPAC name of a molecule. Please give its molecular formula.
user
C21H20N4OS
assistant