  The cross section of top quark pair production is measured in the
$\mathrm{t\bar{t}}$ $\to$
$(\ell\nu_{\ell})(\tau_\mathrm{h}\nu_{\tau})\mathrm{b\bar{b}}$ final state,
where $\tau_\mathrm{h}$ refers to the hadronic decays of the $\tau$ lepton, and
$\ell$ is either an electron or a muon. The data sample corresponds to an
integrated luminosity of 35.9 fb$^{-1}$ collected in proton-proton collisions
at $\sqrt{s} =$ 13 TeV with the CMS detector. The measured cross section is
$\sigma_{\mathrm{t\bar{t}}} =$ 781 $\pm$ 7 (stat) $\pm$ 62 (syst) $\pm$ 20
(lum) pb, and the ratio of the partial width $\Gamma($t $\to$
$\tau\nu_{\tau}$b) to the total decay width of the top quark is measured to be
0.1050 $\pm$ 0.0009 (stat) $\pm$ 0.0071 (syst). This is the first measurement
of the $\mathrm{t\bar{t}}$ production cross section in proton-proton collisions
at $\sqrt{s} =$ 13 TeV that explicitly includes $\tau$ leptons. The ratio of
the cross sections in the $\ell\tau_\mathrm{h}$ and $\ell\ell$ final states
yields a value $R_{\ell\tau_\mathrm{h}/\ell\ell} =$ 0.973 $\pm$ 0.009 (stat)
$\pm$ 0.066 (syst), consistent with lepton universality.
