C# - OpenXML SDK - How to get the IDs of the Master Slides for an empty presentation?

Question

If the presentation is empty or has just one slide the code below (and that's expected) returns 1 ID, but the presentation has 10+ masters. How can I get the full list of existing masters in an any presentation?

        static string[,] GetSlideMasterIds(string templatePath)
            {
                List<string[]> slideMasterIdList = new List<string[]>();

                using (PresentationDocument presentationDocument = PresentationDocument.Open(templatePath, false))
                {
                    // Get the main presentation part
                    PresentationPart presentationPart = presentationDocument.PresentationPart;

                    // Get the slide id list
                    SlideIdList slideIdList = presentationPart.Presentation.SlideIdList;

                    // Iterate through slide ids
                    foreach (SlideId slideId in slideIdList.Elements<SlideId>())
                    {
                        string slideIdValue = slideId.Id.ToString();
                        string relationshipId = slideId.RelationshipId;

                        slideMasterIdList.Add(new string[] { slideIdValue, relationshipId });
                    }
                }

                // Convert the list to a two-dimensional array
                string[,] slideMasterIds = new string[slideMasterIdList.Count, 2];
                for (int i = 0; i < slideMasterIdList.Count; i++)
                {
                    slideMasterIds[i, 0] = slideMasterIdList[i][0]; // Slide ID
                    slideMasterIds[i, 1] = slideMasterIdList[i][1]; // Relationship ID
                }

                return slideMasterIds;
            }

I've also tryed to get the IDs by using the relationship between Slide Layouts and Master Slides, but there was a mess. Any suggestions?