Two blocks of mass 2 kg and 4 kg are connected by a metal wire going over a smooth pulley as shown in figure. The radius of wire is $4.0 \times 10^{-5}$ m and Young's modulus of the metal is $2.0 \times 10^{11}$ N m$^{-2}$. The longitudinal strain developed in the wire is $\dfrac{1}{\alpha \pi}$. The value of $\alpha$ is $\_\_\_\_$. [Use $g = 10$ m s$^{-2}$]