  We measure the pion mass and decay constant on ensembles generated by the
Wuppertal-Budapest Collaboration, and extract the NLO low-energy constants l_3
and l_4 of SU(2) chiral perturbation theory. The data are generated in 2+1
flavor simulations with Symanzik glue and 2-fold stout-smeared staggered
fermions, with pion masses varying from 135 MeV to 400 MeV, lattice scales
between 0.7 GeV and 2.0 GeV, and m_s kept at its physical value. Furthermore,
by excluding the lightest mass points, we are able to test the reliability of
SU(2) chPT as a tool to extrapolate towards the physical point from higher pion
masses.
