Question: There are three real numbers $x$ that are not in the domain of $$f(x) = \frac{1}{1+\frac{1}{1+\frac 1x}}.$$ What is the sum of those three numbers?
A: 0.5
B: 0
C: -1
D: -1.5
Please eliminate two incorrect options first, then think it step by step and choose the most proper one option.