# Differentiability of functional

Let $$\Omega$$ be a bounded domain with Lipschitz border. We define $$G:W^{1,p} \rightarrow\mathbb{R}$$ by $$G(u)=\int_{\partial\Omega}|u|^pd\sigma.$$ How can I show that $$(G'(u),h)=\int_{\partial\Omega}|u|^{p-2}u \, dh\, \sigma$$ for all $$h\in W^{1,p}(\Omega)$$?