  We present the results of the lattice QCD calculation of the average up-down
and strange quark masses and of the light meson pseudoscalar decay constants,
recently performed with Nf=2 dynamical fermions by the ETM Collaboration. 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 f_K=161.7 +- 1.2 +- 3.1 MeV and the ratio f_K/f_pi=1.227 +- 0.009 +-
0.024. From this ratio, by using the experimental determination of Gamma(K ->
mu {bar nu}_mu (gamma))/Gamma(pi -> mu {bar nu}_mu (gamma)) and the average
value of |V_{ud}| from nuclear beta decays, we obtain |V_{us}|=0.2192(5)(45),
in agreement with the determination from K_{l3} decays and the unitarity
constraint.
