  We present the results of a lattice QCD calculation of the average up-down
and strange quark masses and of the light meson pseudoscalar decay constants
with Nf=2 dynamical fermions. The simulation is carried out at a single value
of the lattice spacing with the twisted mass fermionic action at maximal twist,
which guarantees automatic O(a)-improvement of the physical quantities. Quark
masses are renormalized by implementing the non-perturbative RI-MOM
renormalization procedure. Our results for the light quark masses are
m_ud^{msbar}(2 GeV)= 3.85 +- 0.12 +- 0.40 MeV, m_s^{msbar}(2 GeV) = 105 +- 3 +-
9 MeV and m_s/m_ud = 27.3 +- 0.3 +- 1.2. We also obtain fK = 161.7 +- 1.2 +-
3.1 MeV and the ratio fK/fpi=1.227 +- 0.009 +- 0.024. From this ratio, by using
the experimental determination of Gamma(K-> mu nu (gamma))/Gamma(pi -> mu nu
(gamma)) and the average value of |Vud| from nuclear beta decays, we obtain
|Vus|=0.2192(5)(45), in agreement with the determination from Kl3 decays and
the unitarity constraint.
