Q.If $|\vec{a}| = 3$, $|\vec{b}| = 4$, $|\vec{c}| = 5$ and $\vec{a} + \vec{b} + \vec{c} = \vec{0}$, then the value of $\vec{a} \cdot \vec{b} + \vec{b} \cdot \vec{c} + \vec{c} \cdot \vec{a}$ is