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The non-monotonic relationship of Tc with the number of Cu-O planes (n) per unit cell for compounds of Tl2Ba2Can-1CunO2n+4 (n = 1, 2, 3, and 4) is investigated from the perspective of cohesive energy. The unit cell of the Tl superconductors is treated as two blocks: the perovskite block where the Cu-O planes are located and the rock salt block, which is considered as a charge-reservoir. We proposed a model to calculate the combinative energy of the two blocks. It is found that the combinative energy between the two blocks is closely related to the value of Tc. The relation demonstrates an interesting way to understand the nonlinear change of Tc with the number of Cu-O planes in the layered superconductors. This means that the interaction between the two blocks plays an important role in superconductivity.