If $z$ and $w$ are two complex numbers such that $|zw| = 1$ and $\arg(z) - \arg(w) = \dfrac{3\pi}{2}$, then $\arg\left(\dfrac{1 - 2\bar{z}w}{1 + 3\bar{z}w}\right)$ is:

(Here $\arg(z)$ denotes the principal argument of complex number $z$.)