Studies of the decay $B^{0}_{s} \to D^{\mp}_sK^{\pm}$

The decay mode $B^{0}_{s} \to D^{\mp}_s K^{\pm}$ allows for one of the theoretically cleanest time dependent measurements of the CKM angle $\gamma$. This contribution reports the world best branching fraction of this decay relative to the Cabibbo--favoured mode $B^{0}_{s} \to D_{s}^{-}\pi^{+}$ based on data sample of 0.37 $\rm fb^{-1}$ proton--proton collisions at $\sqrt{s} = 7$ TeV collected with the LHCb detector in 2011, resulting in $BR(\BsDK) = (1.90 \pm 0.12^{stat} \pm {{0.13^{syst}}^{+0.12}_{-0.14}}^{f_{s}/f_{d}})\times 10^{-4}$.