If $y=y(x)$ is the solution of the differential equation $\frac{\mathrm{dt}}{\mathrm{dx}}=(\tan \mathrm{x}-\mathrm{y}) \sec ^2 \mathrm{x}, \mathrm{x} \in\left(-\frac{\pi}{2}, \frac{\pi}{2}\right)$, such that $\mathrm{y}(0) =0$, then $\mathrm{y}\left(-\frac{\pi}{4}\right)$ is equal to :
