  The difference of actions between nearly lying unstable ``8'' and ``0'' like
trajectories in two dimensions is computed by methods of chaotic dynamics. We
found it to be equal to $pY\Theta/2$, where $\Theta$ is angle between paths at
cross, $Y$ is distance between paths, and $p$ is momentum. This kind of
periodic trajectories contributes $\tau^2$ (interference) term to spectral
form-factor.
