Let $f\left(x\right)=\left|\right|x|-1|$,  then then number of points where $f\left(x\right)$is not differentiable is______

$y=\left\{\begin{array}{llc}-\left|x\right|+1\text{ if }& \left|x\right|<1& (-1<x<1)\\ \left|x\right|-1\text{ if }& \left|x\right|\ge 1& (x\le \text{ lor }x\ge 1)\end{array}\right.$

$=\left\{\begin{array}{ccc}-x-1& \text{ if }& x\le -1\\ x+1& \text{ if }& -1<x<0\\ -x+1& \text{ if }& 0\le x<1\\ x-1& \text{ if }& x\ge 1\end{array}\right.$