IMO 1990 Problem 2

Let $n \geq 3$ and consider a set $E$ of $2n - 1$ distinct points on a circle. Suppose that exactly $k$ of these points are to be colored black. Such a coloring is good if there is at least one pair of black points such that the interior of one of the arcs between them contains exactly $n$ points from $E$ . Find the smallest value of $k$ so that every such coloring of $k$ points of $E$ is good.