  There are multiple techniques to determine the chemical composition of the
ultra-high-energy cosmic rays. While most of the methods are primarily
sensitive to the average atomic mass, it is challenging to discriminate between
the two lightest elements: proton and helium. In this paper, the
proton-to-helium ratio in the energy range $10^{18.0} \mbox{eV}$ to $10^{19.3}
\mbox{eV}$ is estimated using the tail of the distribution of the depth of the
shower maximum $X_{\mathrm{max}}$. Using the exponential decay scale $\Lambda$
measured by the Pierre Auger Observatory and the Telescope Array experiment we
derive the 68\%\,CL constraints on the proton-to-helium ratio
$\mathrm{p}/\mathrm{He} > 7.3 $ and $\mathrm{p}/\mathrm{He} > 0.43 $ for
$10^{18.0} < E < 10^{18.5}$ eV and $10^{18.3} < E < 10^{19.3}$ eV
correspondingly. It is shown that the result is conservative with respect to
the admixture of heavier elements. We evaluate the impact of the hadronic
interaction model uncertainty. The implications for the astrophysical models of
the origin of cosmic rays and the safety of the future colliders are discussed.
