A short proof of Kneser's addition theorem for abelian groups

Martin Kneser proved the following addition theorem for every abelian group $G$. If $A,B\subseteq G$ are finite and nonempty, then $|A+B| \ge |A+K| + |B+K| - |K|$where$K = {g \in G \mid g+A+B = A+B }$. Here we give a short proof of this based on a simple intersection union argument.