Chemical Equilibrium

For the reaction $\mathrm{N}_2 \mathrm{O}_4(g) \rightleftharpoons 2 \mathrm{NO}_2(g)$, the correct relation between degree of dissociation $(\alpha)$ of $\mathrm{N}_2 \mathrm{O}_4(g)$ and equilibrium constant, $K_p$ is ( $p=$ total pressure of mixture)