Let S be the set of all solutions of the equation cos^-1(2 x)-2 cos^-1(√(1-x^2))=π, x ∈[-1/2, 1/2]. Then Σ_x ∈ S 2 sin^-1(x^2-1) is equal to

Let $\displaystyle S$ be the set of all solutions of the equation $\displaystyle \cos ^{-1}(2 x)-2 \cos ^{-1}\left(\sqrt{1-x^2}\right)=\pi$, $\displaystyle x \in\left[-\frac{1}{2}, \frac{1}{2}\right]$. Then $\displaystyle \sum_{x \in S} 2 \sin ^{-1}\left(x^2-1\right)$ is equal to