A thin equiconvex lens is placed on a plane mirror as shown in figure. A point object O is placed on the principal axis of the lens at a distance of 20 cm from the lens. It is observed that after refraction through the lens, reflection at the mirror and again refraction through the lens, the final image coincides with the object. If radius of curvature of the curved surfaces of the lens is 20 cm, refractive index of its material is