A square plate $\text{O-P-Q-R}$ of a linear elastic material with sides $1.0\text{ m}$ is loaded in a state of plane stress. Under a given stress condition, the plate deforms to a new configuration $O-{P}'-{Q}'-{R}'$ as shown in the figure (not to scale). Under the given deformation, the edges of the plate remain straight.

The horizontal displacement of the point $\left ( \text{0.5 m, 0.5 m} \right )$ in the plate $\text{O-P-Q-R}$ (in $\text{mm}, \textit{round off to one decimal place}$) is __________________