$x,y$满足约束条件$\left\{\begin{array}{l}\begin{array}{c}x+y-2\le 0\\ x-2y-2\le 0\\ 2x-y+2\ge 0\end{array}\\ \end{array}\right.$，若$z=y-ax$取得最大值的最优解不唯一，则实数$a$的值为

$\frac{1}{2}或-1$ $2或\frac{1}{2}$ $2或1$ $2或-1$