 Solution

Let the bunch tail have the higher energy so that it catches the head. For an ultrarelativistic proton, $\delta v/v\simeq (\delta E/E)/({\beta }^2{\gamma }^2)$. A drift of length $S$ then changes the bunch length by approximately $S\delta v/v$. Requiring that to match the initial length givesTaking $\gamma \simeq 62.5/0.938\simeq 66.6$, $\beta \simeq 1$, and the given lengths yields $\Delta E/E\simeq 0.023$, about $2.3\%$. Treating $62.5$ GeV as kinetic rather than total energy changes that by only a few percent.

This is the linear ballistic compression requirement; a real finite-emittance bunch, nonlinear velocity-energy relation, uncorrelated energy spread and collective fields prevent exact zero length. The chirp is a correlated full head-to-tail spread, not automatically an rms energy spread.