NL-NWO 1972 Problem 2

Prove that there exists exactly one function $ƒ$ which is defined for all $x \in R$ , and for which holds: $\bullet$ $x \le y \Rightarrow f(x) \le f(y)$ , for all $x, y \in R$ , and $\bullet$ $f(f(x)) = x$ , for all $x \in R$ .