Matrices and Determinants

Let A = \left( {\matrix{ {\cos \alpha } & { - \sin \alpha } \cr {\sin \alpha } & {\cos \alpha } \cr } } \right), ($\alpha$ $\in$ R) such that {A^{32}} = \left( {\matrix{ 0 & { - 1} \cr 1 & 0 \cr } } \right) then a value of $\alpha$ is