I know it should be easy but I'm trying to determine the complexity of the following variant of Subset Sum.
Given a subset $S$ of positive integers and integers $k>0$ and $N>0$, is there a subset $T\subset S$ such that $|T|=k$ and the members of $T$ sum to $N$ ?
All of the formulations of subset sum that I've seen don't specify $k$ so I'm wondering if this problem can be solved in polynomial time. If $k$ is fixed for all instances, then I know that the problem is in P and solvable by brute force in $O(n^k)$ time. However, I'm allowing $k$ to vary from instance to instance.