  We study the rare decays $B_s^0\to D^\pm \pi^\mp$ and $\bar B_s^0\to D^\mp
\pi^\pm$, which can occur only via annihilation type $W$ exchange diagrams in
the standard model. The time-dependent decay rates of the four channels can
provide four CP parameters, which are experimentally measurable. We show that
the CKM angle $\phi_3=\gamma$ can be determined from these parameters without
any theoretical model dependence. These channels can be measured in future LHCb
experiments to provide a clean way for $\gamma$ measurement.
