If this were ancient times the Greeks would spend endless hours proving this whilst the Romans would just fire up VB VI and get an answer.

Dim a As Integer, b As Integer, c As Integer, d As Integer, n As Integer

n = II For b = n To I Step -I For c = n To I Step -I For d = n To I Step -I n = n + I Debug.Print n Next d Next c Next b

And the answer is MMXCVIII