Excel find nth occurrence of a string within another

Function FindN(sFindWhat As String, _
sInputString As String, N As Integer) As Integer
Dim J As Integer

Application.Volatile
FindN = 0
For J = 1 To N
FindN = InStr(FindN + 1, sInputString, sFindWhat)
If FindN = 0 Then Exit For
Next
End Function