If $A = \begin{bmatrix} \cos 2\theta & -\sin 2\theta \\ \sin 2\theta & \cos 2\theta \end{bmatrix}$ and $A + A^T = I$, where $I$ is the unit matrix of $2 \times 2$ and $A^T$ is the transpose of $A$, then the value of $\theta$ is equal to.