You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
1-[3-(1,2,3,5,6,7,8,8a-octahydroindolizin-7-yl)-1H-indol-5-yl]-3-butylthiourea is the IUPAC name of a molecule. Please give its molecular formula.
user
C21H30N4S
assistant