For a suitably chosen real constant a , let a function, $\mathrm{f}: \mathrm{R}-\{-\mathrm{a}\} \rightarrow \mathrm{R}$ be defined by $f(x)=\frac{a-x}{a+x}$. Further suppose that for any real number $\mathrm{x} \neq-\mathrm{a}$ and $\mathrm{f}(\mathrm{x}) \neq-\mathrm{a}$, $(\mathrm{fof})(\mathrm{x})=\mathrm{x}$. Then $f\left(-\frac{1}{2}\right)$ is equal to: