You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
What is the molecular formula of the molecule ethyl (7S)-7-(2-chloro-5-nitrophenyl)-2-methyl-5-propyl-4,7-dihydro-[1,2,4]triazolo[1,5-a]pyrimidine-6-carboxylate ?
user
C18H20ClN5O4
assistant