$\text{ For a suitably chosen real constant a, let a function, }f:R-\{-a\}\to R\text{ be defined by }$

$f\left(x\right)=\frac{a-x}{a+x}\text{ . Further suppose that for any real number }x\ne -a\text{ and }$

$f\left(x\right)\ne -a,\left(\text{ fof }\right)\left(x\right)=x\text{ . Then }f\left(-\frac{1}{2}\right)\text{ is equal to : }$