First observation of the decays Bˉ0→D+K−π+π− and B−→D0K−π+π−.

First observations of the Cabibbo suppressed decays B0bar -> D+ K- pi+ pi- and B- -> D0 K- pi+ pi- are reported using 35 pb^{-1} of data collected with the LHCb detector. Their branching fractions are measured with respect to the corresponding Cabibbo favored decays, from which we obtain B(B0bar -> D+ K- pi+ pi-)/B(B0bar -> D+ pi- pi+ pi-)=(5.9\pm1.1\pm0.5) x 10^{-2} and B(B- -> D0 K- pi+ pi-)/B(B- -> D0 pi- pi+ pi-)=(9.4\pm1.3\pm0.9) x 10^{-2}, where the uncertainties are statistical and systematic, respectively. The B- -> D0 K- pi+ pi- decay is particularly interesting, as it can be used in a similar way to B- -> D0 K- to measure the CKM phase gamma.