If $y = y(x),y \in \left[ {0,{\pi \over 2}} \right)$ is the solution of the differential equation $\sec y{{dy} \over {dx}} - \sin (x + y) - \sin (x - y) = 0$, with y(0) = 0, then $5y'\left( {{\pi \over 2}} \right)$ is equal to ______________.