How to insert page number in existing PDF document ?

//Load the existing PDF document
PdfLoadedDocument loadedDoc = new PdfLoadedDocument("input.pdf");
 
//Set the font.
PdfFont font = new PdfStandardFont(PdfFontFamily.Helvetica, 12f);
 
//Create page number field.
PdfPageNumberField pageNumber = new PdfPageNumberField(font, PdfBrushes.Black);
 
//Create page count field.
PdfPageCountField count = new PdfPageCountField(font, PdfBrushes.Black);
 
//Add the fields in composite fields.
PdfCompositeField compositeField = new PdfCompositeField(font, PdfBrushes.Black, "Page {0} of {1}", pageNumber, count);
 
for (int i = 0; i < loadedDoc.Pages.Count; i++)
{
//Draw the composite field.
compositeField.Draw(loadedDoc.Pages[i].Graphics, new PointF(loadedDoc.Pages[i].Size.Width / 2 - 20, loadedDoc.Pages[i].Size.Height - 20));
}
 
//Save the document.
loadedDoc.Save("Output.pdf");
 
//Close the document.
loadedDoc.Close(true);