The Young's modulus of a rubber string of length $12 \mathrm{cm}$ and density $1.5 \mathrm{kg} {\mathrm{m}}^{-3}$ is $5\times {10}^8 \mathrm{N} {\mathrm{m}}^{-2}$. When this string is suspended vertically, the increase in its length due to its own weight is (take $\mathrm{g}=10 \mathrm{m} {\mathrm{s}}^{-2}$)