Inverse Trigonometric Functions

If \(\cos ^{-1} x-\cos ^{-1} \frac{y}{2}=\alpha\) where \(-1 \leq x \leq 1, -2 \leq y \leq 2, x \leq \frac{y}{2}\), then for all \(x, y, 4 x^2-4 x y \cos \alpha+y^2\) is equal to