A solid cylinder of radius $R$ made of a material with thermal conductivity $K_1$ is surrounded by a cylindrical shell with an inner radius $R$ and an outer radius $2R$, made of a material with thermal conductivity $K_2$. The two ends of the combined system are maintained at different temperatures. There is no loss of heat across the cylindrical surface, and the system is in a steady state. The effective thermal conductivity of the system is