Putnam 1961 A5

Let $\Omega$ be a set of $n$ points, where $n>2$ . Let $\Sigma$ be a nonempty subcollection of the $2^n$ subsets of $\Omega$ that is closed with respect to the unions, intersections and complements. If $k$ is the number of elements of $\Sigma,$ what are the possible values of $k?$