  Partition functions of some two-dimensional statistical models can be
represented by means of Grassmann integrals over loops living on
two-dimensional torus. It is shown that those Grassmann integrals are
topological invariants, which depend only on the winding numbers of the loops.
The fact makes possible to evaluate the partition functions of the models and
the statistical mean values of certain topological characteristics (indices) of
the configurations, which behave as the (topological) order parameters.
