  Complete one-loop parametrization of the linear sigma model is performed and
the phase boundary between first order and crossover transition regions of the
m_pi-m_K-plane is determined using the optimized perturbation theory as a
resummation tool of perturbative series. Away from the physical point the
parameters of the model were determined by making use of chiral perturbation
theory. Along the diagonal m_pi=m_K of the mass--plane we estimate m_pi^c=110
+- 20 MeV. The location of the tricritical point on the m_pi=0 axis is
estimated in the interval m_K^TCP in (1700,1850) MeV.
