Let A = \left[ {\matrix{
1 & { - 2} & \alpha \cr
\alpha & 2 & { - 1} \cr
} } \right] and B = \left[ {\matrix{
2 & \alpha \cr
{ - 1} & 2 \cr
4 & { - 5} \cr
} } \right],\,\alpha \in C. Then the absolute value of the sum of all values of for which det(AB) = 0 is :