  The cross sections for the production of tt-bar bb-bar and tt-bar jj events
and their ratio sigma[tt-bar bb-bar]/sigma[tt-bar jj] are measured using data
corresponding to an integrated luminosity of 2.3 inverse femtobarns collected
in pp collisions at sqrt(s) = 13 TeV with the CMS detector at the LHC. Events
with two leptons (e or mu) and at least four reconstructed jets, including at
least two identified as b quark jets, in the final state are selected. In the
full phase space, the measured ratio is 0.022 +/- 0.003 (stat) +/- 0.006
(syst), the cross section sigma[tt-bar bb-bar] is 4.0 +/- 0.6 (stat) +/- 1.3
(syst) pb and sigma[tt-bar jj] is 184 +/- 6 (stat) +/- 33 (syst) pb. The
measurements are compared with the standard model expectations obtained from a
POWHEG simulation at next-to-leading-order interfaced with PYTHIA.
