The first measurement of the inclusive cross section for top quark pairs (tt) produced in association with
two additional charm jets is presented. The analysis uses the dileptonic final states of tt events produced
in proton-proton collisions at a centre-of-mass energy of 13 TeV. The data correspond to an integrated
luminosity of 41.5 fb−1, recorded by the CMS experiment at the LHC. A new charm jet identification
algorithm provides input to a neural network that is trained to distinguish among tt events with two
additional charm (ttcc), bottom (ttbb), and light-flavour or gluon (ttLL) jets. By means of a template
fitting procedure, the inclusive ttcc, ttbb, and ttLL cross sections are simultaneously measured, together
with their ratios to the inclusive tt + two jets cross section. This provides measurements of the ttcc and
ttbb cross sections of 10.1 ± 1.2 (stat) ± 1.4 (syst) pb and 4.54 ± 0.35 (stat) ± 0.56 (syst) pb, respectively,
in the full phase space. The results are compared and found to be consistent with predictions from two
different matrix element generators with next-to-leading order accuracy in quantum chromodynamics,
interfaced with a parton shower simulation.