You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
2-[(5-propyl-[1,2,4]triazino[5,6-b]indol-3-yl)sulfanyl]-1-(4-prop-2-ynylpiperazin-1-yl)ethanone is the IUPAC name of a molecule. Please give its molecular formula.
user
C21H24N6OS
assistant