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 identified by (1R)-1-(4-fluorophenyl)-N-(2-methylphenyl)-3,4-dihydro-1H-pyrrolo[1,2-a]pyrazine-2-carbothioamide ?
user
C21H20FN3S
assistant