MathNet 1985 — Part 01 Question 46

A move replaces the real numbers $a$, $b$, $c$, $d$ by $a - b$, $b - c$, $c - d$, $d - a$. If $a$, $b$, $c$, $d$ are not all equal, show that at least one of the numbers can exceed $1985$ after a finite number of moves.