Sorry, I'm adding an answer for myself. There may be other faster ways, but it works.
List<int> lst = new List<int>();
foreach (DataGridViewRow row in dg.SelectedRows)
lst.Add(row.Index);
lst.Sort();
int i = lst[lst.Count - 1];
What this means is, add the indices of all the selected rows to List<>, and then sort, then enter the last item from the sorted one List<>.
. Bala R , , ( , ). , . , , .
!