How to sum multiple columns that change sizes?

I'm still struggling to find a solution to this problem. Not only have I searched here on this forum, but elsewhere on the web. The best I can do is in the code below. However, there is an issue with summing up the second set of numbers as the code loops back up to cell F5 and the issue is compounded as it moves onto the other sets of numbers.

I'm sure there are some smart people out there that can solve this easily and without using old counter logic.

Also it would be useful to have a cell at the bottom that provides a Grand Total of all the sums of the columns this code is adding up.

Sub TotalMultiColumns()

    Worksheets("Compare").Activate
    Range("F5").Activate

    Do Until ActiveCell = ""
        n = n + ActiveCell.Value
        ActiveCell.Range("A2").Select
    Loop
        ActiveCell.Value = n
        n = 0
        ActiveCell.Offset(2, 0).Range("A1").Select

End Sub