Employ stiffness matrix approach for the simply supported beam as shown in the figure to calculate unknown displacements/rotations. Take length, $L=8\text{ m}$; modulus of elasticity, $E=3\times 10^{4} \text{N/mm}^{2}$; moment of inertia, $I=225\times 10^{6}\text{mm}^{4}$.
The mid-span deflection of the beam (in $\text{mm}, \textit{round off to integer}$) under $\text{P=100 kN}$ in downward direction will be _________________