Two concentric uniform spherical shells have the same density ρ. The inner shell has radius R₁ (i.e. fills 0 ≤ r’ ≤ R₁) and the outer shell fills R₁ ≤ r’ ≤ R₂. A particle of mass m is placed at distance r from the center, where R₁ < r < R₂. What is the correct expression for the gravitational potential energy of the particle?