7. Let the function $f(x)=x^{2}-x+1$. Define $f^{(n)}(x)$ as follows:
$$
f^{(1)}(x)=f(x), f^{(n)}(x)=f\left(f^{(n-1)}(x)\right) \text {. }
$$

Let $r_{n}$ be the arithmetic mean of all the roots of $f^{(n)}(x)=0$. Then $r_{2015}=$ $\qquad$