Addition of a proton at $\mathrm{C}^{-1}$ results in the formation of tropylium carbocation (an aromatic species with more stability due to delocalisation of $\pi$ electrons). thus, it is the most reactive site towards protonation. Addition of proton at any other carbon atom interupt in the delocalisation of it electrons by disturbing planarity of molecules and herce makes it less stable. Thus, these all are lest reactive towards protonation.