  Using the isospin-dependent relativistic Vlasov-Uehling-Uhlenbeck (RVUU)
model, we study charged pion ($\pi^\pm$) production in Au+Au collisions at
$\sqrt{s_{NN}}=$ 2.4 GeV. By fitting the density dependence of the $\Delta$
resonance production cross section in nuclear medium to reproduce the
experimental $\pi^\pm$ multiplicities measured by the HADES Collaboration, we
obtain a good description of the rapidity distributions and transverse momentum
spectra of $\pi^\pm$ in collisions at various centralities. Some shortcomings
in the description of $\pi^{\pm}$ production may indicate the need for
including the strong potential on $\pi^\pm$ in RVUU, which is at present
absent. We also calculate the proton rapidity distribution in the most central
collisions and compare with the coalescence invariant proton rapidity
distribution extracted from preliminary HADES data.
