In the figure, the load $P=1 \: N$, length $L=1 \: m$, Young’s modulus $E=70 \: GPa$, and the cross-section of the links is a square with dimension $10 \: mm \times 10 \: mm$. All joints are pin joints.
The stress (in $Pa$) in the link $AB$ is ___________
(Indicate compressive stress by a negative sign and tensile stress by a positive sign.)