The axial velocity profile of a laminar, incompressible, and fully-developed flow in a circular pipe of radius $R$ is given as $v_{z}=\dfrac{1}{4 \mu} \dfrac{d p}{d z}\left(r^{2}-R^{2}\right)$, where $\mu, p, z$, and $r$ are dynamic viscosity, pressure, axial coordinate, and radial coordinate, respectively. If the magnitude of shear stress at the pipe wall is given as $\mid \tau_{w}\mid=\dfrac{R}{K} \dfrac{d p}{d z}$, then the value of $K$ is $\_\_\_\_$. (answer in integer)
