Let $$M = \left[ {\matrix{ 0 & { - \alpha } \cr \alpha & 0 \cr } } \right]$, where $\alpha$ is a non-zero real number an $N = \sum\limits_{k = 1}^{49} {{M^{2k}}} $. If $(I - {M^2})N = - 2I$, then the positive integral value of $\alpha$$ is ____________.