A charge $+q$ is placed at each of the points $x = x_0$, $x = 3x_0$, $x = 5x_0$, $\ldots$ on the $x$-axis, and a charge $-q$ is placed at each of the points $x = 2x_0$, $x = 4x_0$, $x = 6x_0$, $\ldots$ to infinity. Here $x_0$ is a positive constant. Take the electric potential at a point due to a charge $q$ at distance $r$ from it to be $q/(4\pi\varepsilon_0 r)$. Then the potential at the origin due to the above system of charges is