You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
1-(3,3a,4,9b-tetrahydrochromeno[4,3-c][1,2]oxazol-1-yl)-2-methyl-3-phenylpropan-1-one is the IUPAC name of a molecule. Please give its molecular formula.
user
C20H21NO3
assistant