Closest point on a circle to (1,2)

Let $C=\{(x,y)\in\mathbb{R}^2:x^2+y^2=12\}$ be a circle in the plane. Let $(a,b)$ be the point on $C$ which minimizes the distance to the point $(1,2)$. Then $b-a$ is __________ (round off to 2 decimal places)