Let $a_1 \ge a_2 \ge ... \ge a_n > 0$ be real numbers. Prove that $$a_1a_2(a_1 - a_2) + a_2a_3(a_2 - a_3) +...+ a_{n-1}a_n(a_{n-1} - a_n) \ge a_1a_n(a_1 - a_n)$$