A given steel has identical yield strength of 700 MPa in uni-axial tension and uni-axial compression. If the steel is subjected to pure shear stress such that the three principal stresses are \( \sigma_1 = \sigma, \sigma_2 = 0, \sigma_3 = -\sigma \) with \( \sigma_1 \geq \sigma_2 \geq \sigma_3 \), then the stress \( \sigma \) in MPa for the initiation of plastic yielding in the steel as per von Mises yield criterion is _________.
\text{[round off to 2 decimal places]}