Sum of Unknown Number of Rows

I've tried this code but doesn't seem to work, any pointers?

 
    Dim Count As Long
    Range("A1").Select
   
        Range(Selection, Selection.End(xlDown)).Select
   
    Count = Selection.Cells.CountA
   
    Range("A1").Select
    
    Selection.End(xlDown).Select
    ActiveCell.Offset(2, 2).Select
    ActiveCell.FormulaR1C1 = "=SUM(R[-'Counta']C:R[-2]C)"