  The doubly charmed baryon decay $\Xi_{cc}^{++} \rightarrow \Xi_{c}^{+}
\pi^{+}$ is observed for the first time, with a statistical significance of
$5.9\sigma$, confirming a recent observation of the baryon in the
$\Lambda_c^{+} K^{-} \pi^{+} \pi^{+}$ final state. The data sample used
corresponds to an integrated luminosity of $1.7\,\mathrm{fb}^{-1}$, collected
by the LHCb experiment in $pp$ collisions at a center-of-mass energy of
$13\mathrm{\,Te\kern -0.1em V}$. The $\Xi_{cc}^{++}$ mass is measured to be
  \begin{equation}\nonumber
  3620.6\pm 1.5~(\text{stat})\pm 0.4~(\text{syst}) \pm
0.3~(\Xi_{c}^{+})~\text{MeV}/\it{c}^{2},
  \end{equation}
  and is consistent with the previous result. The ratio of branching fractions
between the decay modes is measured to be
  \begin{equation}\nonumber
  \frac{\mathcal{B} (\Xi_{cc}^{++} \rightarrow \Xi_{c}^{+} \pi^{+}) \times
\mathcal{B}(\Xi_{c}^{+} \rightarrow pK^{-}\pi^{+})}
  {\mathcal{B} (\Xi_{cc}^{++} \rightarrow \Lambda_c^{+} K^{-} \pi^{+} \pi^{+})
\times \mathcal{B}(\Lambda_c^{+} \rightarrow pK^{-}\pi^{+})}
  = 0.035\pm 0.009~(\text{stat}) \pm 0.003~(\text{syst}).
  \end{equation}
