  The production of four top quarks ($\mathrm{t\bar{t}t\bar{t}}$) is studied
with LHC proton-proton collision data samples collected by the CMS experiment
at a center-of-mass energy of 13 TeV, and corresponding to integrated
luminosities of up to 138 fb$^{-1}$. Events that have no leptons
(all-hadronic), one lepton, or two opposite-sign leptons (where lepton refers
only to prompt electrons or prompt muons) are considered. This is the first
$\mathrm{t\bar{t}t\bar{t}}$ measurement that includes the all-hadronic final
state. The observed significance of the $\mathrm{t\bar{t}t\bar{t}}$ signal in
these final states of 3.9 standard deviations (1.5 expected) provides evidence
for $\mathrm{t\bar{t}t\bar{t}}$ production, with a measured cross section of 36
$^{+12}_{-11}$ fb. Combined with earlier CMS results in other final states, the
signal significance is 4.0 standard deviations (3.2 expected). The combination
returns an observed cross section of 17 $\pm$ 4 (stat) $\pm$ 3 (syst) fb, which
is consistent with the standard model prediction.
