Applying Macro to all worksheets (tabs)

Hi,

I have written a macro to sum column N in a specified worksheet. But I need the code to apply the macro to all the worksheets in my workbook.
I have been searching how to do this to no avail. Can anyone help?

Here is my code:

Sub Sum()

Dim LR As Long
With Sheets("UNIVERSITY OF HELSINKI")
    LR = .Range("N" & Rows.Count).End(xlUp).Row
    .Range("N" & LR + 1).Value = WorksheetFunction.Sum(.Range("N1:N" & LR))
End With
End Sub