Touch Application - Rotation (WPF, VS 2010 c#, Windows 7)

Hi, 

I made a simple 2D image rotation (WPF, VS 2010 c#, Windows 7) on multi-touch tabletop. Currently it is with free rotation depending on fingers movement. Can I make rotation only towards defined angles e.g. (0, 90,180 etc)?

C#

void Window_ManipulationDelta(object sender, ManipulationDeltaEventArgs e)
{

rectsMatrix.RotateAt(e.DeltaManipulation.Rotation, 
             e.ManipulationOrigin.X, 
             e.ManipulationOrigin.Y);
    ....
}
Thank you
Madni