If $y = {\cos ^{ - 1}}\cos (|x| - f(x)),$ where $$ $f(x)\left\{ \begin{array}{l} = 1\,,\,{\rm{if}}\,\,\,x > 0\\ = - 1\,,\,{\rm{if}}\,\,\,x < 0\\= 0\,,\,{\rm{if}}\,\,\,x = 0 \end{array} \right.$, then ${\left. {\frac{{dy}}{{dx}}} \right|_{x = \frac{{5\pi }}{4}}}$ is