Let the lengths of intercepts on $x$-axis and $y$-axis made by the circle $x^2+y^2+a x+2 a y+c=0,(a< 0)$ be $2 \sqrt{2}$ and $2 \sqrt{5}$, respectively. Then the shortest distance from origin to a tangent to this circle which is perpendicular to the line $x+2 y=0$, is euqal to :
