A thin positive lens is applied closely to a concave spherical mirror, the radius of curvature of which is R = 1 m. At a distance a = 20 cm in front of this system, perpendicular to its optical axis, a flat object is located. It turned out that the plane of the object coincides with the plane of the image formed after the passage of light through the lens, reflection from the mirror and secondary passage through the lens. Determine the focal length of the lens.

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