Dim x, y, f, sum As Double
    Dim i As Integer

    x = 1
    y = 3
    sum = 0

    For i = 1 To 13
        f = x / y
        x = y
        y = y + 2
        If i Mod 2 = 1 Then
            f = -f
        End If

        sum = sum + f
    Next i

    Range("A1").Value = sum