US-TST 2007 Problem 2

Let $n$ be a positive integer and let $a_1 \le a_2 \le \dots \le a_n$ and $b_1 \le b_2 \le \dots \le b_n$ be two nondecreasing sequences of real numbers such that $$a_1 + \dots + a_i \le b_1 + \dots + b_i \text{ for every } i = 1, \dots, n$$ and $$a_1 + \dots + a_n = b_1 + \dots + b_n.$$ Suppose that for every real number $m$ , the number of pairs $(i,j)$ with $a_i-a_j=m$ equals the numbers of pairs $(k,\ell)$ with $b_k-b_\ell = m$ . Prove that $a_i = b_i$ for $i=1,\dots,n$ .