8. Find all integer values of the parameter $a$ for which the system $\left\{\begin{array}{l}x-2 y=y^{2}+2, \\ a x-2 y=y^{2}+x^{2}+0.25 a^{2}\end{array}\right.$. has at least one solution. In your answer, specify the sum of the found values of the parameter $a$.