Let \( A \) be a \( 3\times 3 \) real matrix such that \( A^{2}(A-2I)-4(A-I)=0 \), where \( I \) and \( O \) are the identity and null matrices, respectively. If \( A^{5}=\alpha A^{2}+\beta A+\gamma I \), where \( \alpha,\beta \) and \( \gamma \) are real constants, then \( \alpha+\beta+\gamma \) is equal to: