Numerical Methods: Euler to Runge–Kutta · Easy problem

Euler's method is applied to the system $x' = y$, $y' = -x$ with $x(0) = 1$, $y(0) = 0$ and step $h = 0.1$. After one step the point is $(x_1, y_1)$. Find $x_1^2 + y_1^2$. Express your answer as a common fraction.