Let A = [0, 2, -3; -2, 0, 1; 3, -1, 0] and B be a matrix such that B ( I - A )= I + A. Then the sum of the diagonal elements of B^T B is equal to ____

Let $\displaystyle \mathrm{A}=\left[\begin{array}{ccc}0 & 2 & -3 \\ -2 & 0 & 1 \\ 3 & -1 & 0\end{array}\right]$ and B be a matrix such that $\displaystyle \mathrm{B}(\mathrm{I}-\mathrm{A})=\mathrm{I}+\mathrm{A}$. Then the sum of the diagonal elements of $\displaystyle B^T B$ is equal to $\displaystyle \_\_\_\_$