Distance between centers of two spheres A and B, each of radius R as shown in figure. Sphere B has a spherical cavity of radius R/2 such that distance of centre of cavity is (r – R/2)  from the centre of sphere A and R/2 from the centre of sphere B. Di-electric constant of material of each sphere is K = 1 and material of each sphere has a uniform charge density $\rho$  per unit volume. The interaction potential energy of the two spheres is found to be  $\frac{\pi {\rho }^2{R}^6(7r-4R)}{K{\epsilon }_{0}r(2r-R)}$. The value of K is