If $\vec{a}, \vec{b},$ and $\vec{c}$ are unit vectors such that $\vec{a}+\vec{b}+\vec{c}=\vec 0$, then value of $\vec{a}.\vec{b}+\vec{b}.\vec{c}+\vec{c}.\vec{a}$ is :

0 $-\frac{3}{2}$ $\frac{3}{2}$ 2

$-\frac{3}{2}$