How to get embranchment index?

IDictionary<int, IList<int>> secondlevel = new Dictionary<int, IList<int>>();
IDictionary<int, IList<int>> thirdlevel = new Dictionary<int, IList<int>>();
 secondlevel.Add(1, new List<int>(new int[] { 1, 2 }));
 secondlevel.Add(2, new List<int>(new int[] { 3 }));
 secondlevel.Add(3, new List<int>(new int[] { 4, 5, 6 }));

 thirdlevel.Add(1, new List<int>(new int[] { 5, 6, 7, 8, 20, 13, 24 }));
 thirdlevel.Add(2, new List<int>(new int[] { 23 }));
 thirdlevel.Add(3, new List<int>(new int[] { 22, 25, 26, 27 }));
 thirdlevel.Add(4, new List<int>(new int[] { 19, 28, 29, 30, 31, 38, 33 }));
 thirdlevel.Add(5, new List<int>(new int[] { 32, 35 }));
 thirdlevel.Add(6, new List<int>(new int[] { 34, 36, 37 }));