The decay-time-dependent CP asymmetry in B-0 -> (DD -/+)-D-+/- decays is mea- sured using a data set corresponding to an integrated luminosity of 9 fb(-1) recorded by the LHCb detector in proton-proton collisions at centre-of-mass energies of 7, 8 and 13 TeV. The CP parameters are measured asSDD=-0.861 +/- 0.077 +/- 0.019,Delta SDD=0.019 +/- 0.075 +/- 0.012,CDD=-0.059 +/- 0.092 +/- 0.020,Delta CDD=-0.031 +/- 0.092 +/- 0.016,ADD=0.008 +/- 0.014 +/- 0.006. The analysis provides the most precise single measurement of CP violation in this decay channel to date. All parameters are consistent with their current world average values.