If $f:R\to R$ is defined by$f\left(x\right)=$ $\left\{\begin{array}{l}\begin{array}{ccc}\frac{{\displaystyle 2\sin x-\sin 2x}}{{\displaystyle 2x\cos x}}& if& x\ne 0\end{array}\\ \begin{array}{ccc} a& if& x=0\end{array}\end{array}\right.,$then the value of $a$ so that $f$ is continuous at 0 is