  Pure Yang-Mills SU(N) theory is studied in four dimensional space and Landau
gauge by a double perturbative expansion based on a massive free-particle
propagator. By dimensional regularization, all diverging mass terms cancel
exactly in the double expansion, without the need to include mass counterterms
that would spoil the symmetry of the original Lagrangian. The emerging
perturbation theory is safe in the infrared and shares the same behaviour of
the standard perturbation theory in the UV. At one-loop, gluon and ghost
propagators are found in excellent agreement with the data of lattice
simulations and an infrared-safe running coupling is derived. A natural scale
m=0.6-0.7 GeV is extracted from the data for N=3.
