ellipse

The equation of an ellipse is
$$\frac{x^2}{a^2} + \frac{y^2}{b^2} = 1,$$
A circle is drawn so that it is contained in the ellipse, its center lies on the $x$-axis, and it touches the ellipse only at the point $(a,0).$  If a = b = 5, find the radius of the circle.