Macro taking a lot of time

Guys,

I have the following code that is taking a lot of time. I am a novice in vba and built this code with the help of Google. Please help me to optimize this.
Thanks.

Sub BSERealtyInflationReturns()
Dim RowCount As Integer
Dim RowCount1 As Integer
Dim ColumnCount As Integer

    Sheets("BSERealtyIndexSheet2").Select
    Range("C2").Select
    
      
    Range("D1").Select
    Selection.Value = 100
    Selection.Offset(1, 0).Select
    
    RowCount = Application.CountA(Range("A:A"))
    RowCount1 = 0 - RowCount + 1
    ColumnCount = 1
    
    Do Until Selection.Offset(0, -3) = ""
    
    
    
    Do Until Selection.Offset(0, -3) = ""
    
        Selection.Value = Selection.Offset(-1, 0) * (Selection.Offset(0, -1 * ColumnCount) + 1) + 100
        Selection.Offset(1, 0).Select
        
    Loop
    
    Selection.Offset(RowCount1, 1).Select
    ColumnCount = ColumnCount + 1
    RowCount1 = RowCount1 + 1
    Loop
    

End Sub