Write the argument of −i.

Let $z=-i$

Then, $\operatorname{Re}(z)=0, \operatorname{Im}(z)=-1$

Since, the point $(0,-1)$ representing $z=0-i$ lies on negative direction of imaginary axis.

Therefore,

$\arg (z)=\frac{-\pi}{2}$ or $\frac{3 \pi}{2}$