Real Eigenvalues From Uniform Entries

Let

$M = \begin{bmatrix} a & b \ c & d \end{bmatrix}$

where $a, b, c, d$ are independent and uniform on $(-r, r)$ for some $r > 0$. The eigenvalues of $M$ are real exactly when the discriminant of its characteristic polynomial is non-negative, that is when

$(a-d)^2 + 4bc \geq 0$

What is that probability when $r = 1$? Answer format: round to 3 decimal places.