Get all text in all slides in Power Point (.pptx) using OpenXML in ASP.Net

What is (A.Text) how to refrence pls help.

Refrence - https://docs.microsoft.com/en-us/office/open-xml/how-to-get-all-the-text-in-all-slides-in-a-presentation

    public static int CountSlides(string presentationFile)
    {
        // Open the presentation as read-only.
        using (PresentationDocument presentationDocument = PresentationDocument.Open(presentationFile, false))
        {
            // Pass the presentation to the next CountSlides method
            // and return the slide count.
            return CountSlides(presentationDocument);
        }
    }

    // Count the slides in the presentation.
    public static int CountSlides(PresentationDocument presentationDocument)
    {
        // Check for a null document object.
        if (presentationDocument == null)
        {
            throw new ArgumentNullException("presentationDocument");
        }

        int slidesCount = 0;

        // Get the presentation part of document.
        PresentationPart presentationPart = presentationDocument.PresentationPart;
        // Get the slide count from the SlideParts.
        if (presentationPart != null)
        {
            slidesCount = presentationPart.SlideParts.Count();
        }
        // Return the slide count to the previous method.
        return slidesCount;
    }

    public static void GetSlideIdAndText(out string sldText, string docName, int index)
    {
        using (PresentationDocument ppt = PresentationDocument.Open(docName, false))
        {
            // Get the relationship ID of the first slide.
            PresentationPart part = ppt.PresentationPart;
            OpenXmlElementList slideIds = part.Presentation.SlideIdList.ChildElements;

            string relId = (slideIds[index] as SlideId).RelationshipId;

            // Get the slide part from the relationship ID.
            SlidePart slide = (SlidePart) part.GetPartById(relId);

            // Build a StringBuilder object.
            StringBuilder paragraphText = new StringBuilder();

            // Get the inner text of the slide:
            IEnumerable<A.Text> texts = slide.Slide.Descendants<A.Text>();
            foreach (A.Text text in texts)
            {
                paragraphText.Append(text.Text);
            }
            sldText = paragraphText.ToString();
        }              
    }