The sum of all possible values of \( \theta \in [-\pi, 2\pi] \), for which \(\displaystyle \frac{1 + i\cos\theta}{1 - 2i\cos\theta} \) is purely imaginary, is equal to:
\( 4\pi \)
\( 3\pi \)
\( 5\pi \)
\( 2\pi \)
