Numerical Methods: Euler to Runge–Kutta · Hard problem

Use two steps of the improved Euler (Heun) method with $h = \dfrac{1}{2}$ on $y' = t - y$, $y(0) = 1$, to estimate $y(1)$. (Heun: $k_1 = f(t_n, y_n)$, $k_2 = f(t_n + h, y_n + h k_1)$, $y_{n+1} = y_n + \dfrac{h}{2}(k_1 + k_2)$.) Express your answer as a common fraction.