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// For complete examples and data files, please go to https://github.com/groupdocs-watermark/GroupDocs.Watermark-for-.NET
using (SlidesDocument doc = Document.Load<SlidesDocument>(Utilities.MapSourceFilePath (FilePath)))
{
    // Add text watermark
    TextWatermark textWatermark = new TextWatermark("Test watermark", new Font("Arial", 8));
    doc.Slides[0].AddWatermark(textWatermark);

    // Add image watermark
    using (ImageWatermark imageWatermark = new ImageWatermark(@"D:\logo.jpg"))
    {
        doc.Slides[1].AddWatermark(imageWatermark);
    }
    doc.Save();
}