The elastic potential energy stored in a steel wire of length $20 \mathrm{~m}$ stretched through $2 \mathrm{~cm}$ is $80 \mathrm{~J}$. The cross sectional area of the wire is __________ $\mathrm{mm}^{2}$.

$\left(\right.$ Given, $\left.y=2.0 \times 10^{11} \mathrm{Nm}^{-2}\right)$