Let $M = \left[\begin{array}{cc} -4 &1\cr -4 &4 \end{array}\right]$. Find $c_1$ and $c_2$ such that $M^2 + c_1 M + c_2 I_2 = 0$, where $I_2$ is the identity $2\times 2$ matrix.

$c_1=$ ,
$c_2=$ .