A convex lens of focal length $$20 \text{ cm}$$ is placed in front of convex mirror with principal axis coinciding each other. The distance between the lens and mirror is $$10 \text{ cm}$$. A point object is placed on principal axis at a distance of $$60 \text{ cm}$$ from the convex lens. The image formed by combination coincides the object itself. The focal length of the convex mirror is ______ cm.