Two persons pull a wire towards themselves. Each person exerts a force of 200 N on the wire. Young's modulus of the material of wire is $1 \times 10 ^ { 11 } \mathrm {~N} \mathrm {~m} ^ { - 2 }$. Original length of the wire is 2 m and the area of cross section is $2 \mathrm {~cm} ^ { 2 }$. The wire will extend in length by $\_\_\_\_$ $\mu \mathrm { m }$.