The LHCb experiment is designed to perform flavour physics measurements at the Large Hadron Collider. Using data collected during the 2010 run, we reconstruct a sample of $H_brightarrow h^+h^-$ decays, where $H_b$ can be either a $B^0$ meson, a $B_s^
0$ meson or a $Lambda_b$ baryon, while $h$ and $h^prime$ stand for $pi$, $K$ or $p$. We provide preliminary values of the direct $mathcal{CP}$ asymmetries of the neutral $B^0$ and $B_s^0$ mesons $A_{CP} (B^0rightarrow K^+pi^-) = -0.074 pm 0.033mathrm{(stat.)} pm 0.008mathrm{(syst.)}$ and $A_{CP}(B_s^0rightarrowpi^+K^-)=0.15 pm 0.19mathrm{(stat.)} pm 0.02mathrm{(syst.)}$.
