You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
1-[4-[(3aR,4R,6aS)-4-[methyl-(5-methylpyrazin-2-yl)amino]-3,3a,4,5,6,6a-hexahydro-1H-cyclopenta[c]pyrrol-2-yl]piperidin-1-yl]ethanone is the IUPAC name of a molecule. Please give its molecular formula.
user
C20H31N5O
assistant