K is a field and $char(K)=p$, $K(s,t)$ is the field of rational functions in variable $s$ and $t$. Now $K(s^{p},t^{p})\subseteq K(s,t)$. Prove that $K(s,t)$ is a simple field extension of $K(s^{p},t^{p})$ ( which means $K(s,t)=K(s^{p},t^{p})(u)$ ).
Right now I can prove that the degree of this extension is $p^{2}$ and I want to prove by contradiction. But I don't know how to derive a contradiction.