You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
(1aR,3S,3aR,7aS)-3-methyl-2,3a,4,5,6,7-hexahydro-1aH-phosphindolo[3,3a-b]oxirene 3-oxide is the IUPAC name of a molecule. Please give its molecular formula.
user
C9H15O2P
assistant