If \( \vec{a}, \vec{b} \) and \( \vec{c} \) are unit vectors such that \( \vec{a}+\vec{b}+\vec{c}=0 \) , then the value of \( \vec{a} \cdot \vec{b}+\vec{b} \cdot \vec{c}+\vec{c} \cdot \vec{a} \) is