Arithmetic operation resulted in an overflow. blowfish algorythm

Dim bytearray() As Byte = System.IO.File.ReadAllBytes("C:\Users\Fery Ferdiansyah\Desktop\asd\asd.txt")
    Dim key() As Byte = System.Text.Encoding.UTF8.GetBytes("swagger")
    Dim bf As New blowfish2(key)

Private Function F(ByVal x As UInteger) As UInteger
        Dim a As UShort
        Dim b As UShort
        Dim c As UShort
        Dim d As UShort
        Dim y As UInteger

        d = CUShort((x And &HFF))
        x >>= 8
        c = CUShort((x And &HFF))
        x >>= 8
        b = CUShort((x And &HFF))
        x >>= 8
        a = CUShort((x And &HFF))
        y = S(0, a) + S(1, b)
        y = y Xor S(2, c)
        y = y + S(3, d)

        Return y
    End Function