A steel rod has a radius of $20 \mathrm{~mm}$ and a length of $2.0 \mathrm{~m}$. A force of $62.8 ~\mathrm{kN}$ stretches it along its length. Young's modulus of steel is $2.0 \times 10^{11} \mathrm{~N} / \mathrm{m}^{2}$. The longitudinal strain produced in the wire is _____________ $\times 10^{-5}$