Given that the inverse trigonometric function assumes principal values only. Let $x$, $y$ be any two real numbers in $[-1,1]$ such that $\cos^{-1}x - \sin^{-1}y = \alpha$, $\dfrac{-\pi}{2}\leq\alpha\leq\pi$. Then, the minimum value of $x^2+y^2+2xy\sin\alpha$ is