A steel wire $2.0\ \text{m}$ long, with a cross-sectional area of $1.0\ \text{mm}^2$, is stretched slowly by $1.0\ \text{mm}$. Young's modulus of steel is $2.0 \times 10^{11}\ \text{N/m}^2$. The elastic energy stored in the wire, in millijoules, is ____.