Numerical Methods: Euler to Runge–Kutta · Easy problem

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