You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
For the molecule named (5R)-7-ethyl-N-[(2S)-2-phenylpropyl]-1-oxa-2,7-diazaspiro[4.4]non-2-ene-3-carboxamide by IUPAC nomenclature, provide its molecular formula.
user
C18H25N3O2
assistant