For Each Next Loop

This is my first time using the "For Each Next" loop. I want to subtotal values in each sheet. Unless I make the worksheet active in the loop the code kept summing the same worksheet.

The question is do I have to activate the next sheet in the loop. Here is the code I have been using:

Sub SumCost()
Dim WS As Worksheet
Dim FinalRow As Integer
For Each WS In ActiveWorkbook.Worksheets
WS.Activate
FinalRow = Cells(Rows.Count, 11).End(xlUp).Row
Range("k" & FinalRow + 1).Formula = "=sum(k2:k" & FinalRow & ")"
Range("A" & FinalRow + 1).Value = "Total"
Next WS
End Sub