IR-MO 2005 Problem 6

Suppose $A\subseteq \mathbb R^m$ is closed and non-empty. Let $f:A\to A$ is a lipchitz function with constant less than 1. (ie there exist $c<1$ that $|f(x)-f(y)|<c|x-y|,\ \forall x,y \in A)$ . Prove that there exists a unique point $x\in A$ such that $f(x)=x$ .