Observation of the doubly cabibbo-suppressed decay D_{s};{+}-->K;{+}K;{+}pi;{-}

We report the first observation of the doubly Cabibbo-suppressed decay D_{s};{+}-->K;{+}K;{+}pi;{-} using 605 fb;{-1} of data collected with the Belle detector at the KEKB asymmetric-energy e;{+}e;{-} collider. The branching ratio with respect to its Cabibbo-favored counterpart B(D_{s};{+}-->K;{+}K;{+}pi;{-})/B(D_{s};{+}-->K;{+}K;{-}pi;{+}) is (0.229+/-0.028+/-0.012)%, where the first uncertainty is statistical and the second is systematic. We also report a significantly improved measurement of the doubly Cabibbo-suppressed decay D;{+}-->K;{+}pi;{+}pi;{-}, with a branching ratio B(D;{+}-->K;{+}pi;{+}pi;{-})/B(D;{+}-->K;{-}pi;{+}pi;{+})=(0.569+/-0.018+/-0.014)%.