Let $n,k$ be positive integers. Is there a closed form of the sum
$\sum_{s=0}^{k} \binom{n}{s} \binom{s}{k-s}\text{?}$
By that I mean a representation which is free of sums and hypergeometric functions or alike.
Combinatoric interpretation: This is the number of possibilities to distribute $k$ balls in $n$ urns, where each urn has at most $2$ balls.