Extracting backgound image from powerpoint slide using c#

PowerPoint.Application objApp;

PowerPoint.Presentations objPresSet;

PowerPoint._Presentation objPres;

objApp = new PowerPoint.Application();

objPresSet = objApp.Presentations;

objPres = objPresSet.Open("G:\\temp.ppt", Microsoft.Office.Core.MsoTriState.msoFalse, Microsoft.Office.Core.MsoTriState.msoTrue, Microsoft.Office.Core.MsoTriState.msoTrue);

int SlideCount = objPres.Slides.Count;

int FirstSlideId = objApp.ActiveWindow.Selection.SlideRange.SlideID;

for (int j = FirstSlideId; j < FirstSlideId + SlideCount; j++)

{
      objPres.Slides.FindBySlideID(j).Master.Background.Export("C:\\img" + j + ".jpg",
      Microsoft.Office.Interop.PowerPoint.PpShapeFormat.ppShapeFormatJPG, 800, 600,
      Microsoft.Office.Interop.PowerPoint.PpExportMode.ppScaleToFit);     // Exporting Slide Background                                                          

}