F[((p -> ?) -> ?) -> p]          |         T[(p -> ?) -> ?]         F[p] +--------+--------+F[p -> ?]          T[?] |                 |T[p]            CONTRADICTION (? is true)F[?] |CONTRADICTION (T[p], F[p])

Hilbert-style proofs can be found here or here.

 1. F[(p -> q) -> ((q -> r) -> (p -> r))]              |                       // from 1          2. T[p -> q]          3. F[(q -> r) -> (p -> r)]              |                       // from 3          4. T[q -> r]          5. F[p -> r]              |                       // from 5          6. T[p]          7. F[r]     +--------+--------+              // from 28a. F[p]          8b. T[q]     X        +--------+--------+     // from 4         9a. F[q]          9b. T[r]              X                 X

A Hilbert-style proof can be found here.