Consider the stepped bar made with a linear elastic material and subjected to an axial load of $1$ kN, as shown in the figure.
Segments $1$ and $2$ have cross-sectional area of $100 \: mm^2$ and $60 \: mm^2$, Young’s modulus of $2 \times 10^5$ MPa, and length of $400$ mm and $900$ mm, respectively. The strain energy (in N-mm, up to one decimal place) in the bar due to the axial load is __________