Based on the equilibrium constant for the reaction below :

$2{\mathrm{SO}}_3\left(g\right)\rightleftharpoons 2{\mathrm{SO}}_2\left(g\right)+{\mathrm{O}}_2\left(g\right)K=1.8\times {10}^{-5}$

What is the equilibrium constant for the reaction 

${\mathrm{SO}}_2\left(g\right)+\frac{1}{2}{\mathrm{O}}_2\left(g\right)\rightleftharpoons {\mathrm{SO}}_3\left(g\right)K=?$