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