6. Let real numbers $a, b, c$ satisfy $a^{2}+b^{2}+c^{2}=1$, and let the maximum and minimum values of $ab+bc+ca$ be $M, m$ respectively. Then $M-m=$ $\qquad$ .
6. $\frac{3}{2}$.

Obviously, $M=1, m=-\frac{1}{2}$.