Numerical Methods: Euler to Runge–Kutta · Hard problem

The problem $y' = 1 + y^2$, $y(0) = 0$ has exact solution $y = \tan t$. Apply one step of Heun's method ($k_1 = f(t_0, y_0)$, $k_2 = f(t_0 + h,\ y_0 + hk_1)$, $y_1 = y_0 + \dfrac{h}{2}(k_1 + k_2)$) with $h = \dfrac{1}{2}$. Express the estimate of $y\left(\dfrac{1}{2}\right)$ as a common fraction.