[Paraview] How to get data from desired time step

[Sergei D.]

Hello.
I have time series of VTU files with the coordinates of spheres and radii.
I heed calculate the velocity of these spheres from ProgrammableFiler.
So, I need data from the previous time step.

I get current time step as follow:

ind=self.GetInput()
iin=ind.GetInformation()
curTimeStep=iin.Get(ind.DATA_TIME_STEPS(),0)

Now, how can I get spheres coordinates from curTimeStep-1?

Best regards, Sergei D.