Using untagged $B^0\to {\mathrm{DK}}_S$ to determine $\gamma$

It is shown that the weak phase $\gamma \equiv \arg \left(-V_{\mathrm{ud}}{V}_{\mathrm{ub}}^{*}{V}_{\mathrm{cb}}{V}_{\mathrm{cd}}^{*}\right)$ can be determined using only untagged decays $B^0/B^0\to {\mathrm{DK}}_S.$ In order to reduce the uncertainty in $\gamma ,$ we suggest combining information from $B^{\pm }\to {\mathrm{DK}}^{\pm }$ and from untagged $B^0$ decays, where the D meson is observed in common decay modes. Theoretical assumptions, which may further reduce the statistical error, are also discussed.