Nonlinear Systems & Linearization · Medium problem

The system $x' = y,\; y' = -x + x^2$ conserves $E = \dfrac{y^2}{2} + \dfrac{x^2}{2} - \dfrac{x^3}{3}$. Orbits near the origin are closed; the boundary of that family of closed orbits is the level set of $E$ through the saddle point. What is the value of $E$ at the saddle?