You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
3-(1,1a,6,6a-tetrahydrocyclopropa[a]inden-1-ylmethyl)-1-(1-hydroxypropan-2-yl)-1-(2-hydroxypropyl)urea is the IUPAC name of a molecule. Please give its molecular formula.
user
C18H26N2O3
assistant