Let $$S = \{n \in N, \begin{pmatrix} 0 & i \\ 1 & 0 \end{pmatrix}^n \begin{pmatrix} a & b \\ c & d \end{pmatrix}= \begin{pmatrix} a & b \\ c & d \end{pmatrix}$$ $$\forall a, b, c, d \in R$$, where $$i = \sqrt{-1}\}$$. Then the number of 2-digit numbers in the set $$S$$ is ___.