Školjka

The sequence $a_1,a_2,\dots$ of integers satisfies the conditions: (i) $1\le a_j\le2015$ for all $j\ge1$,\\ (ii) $k+a_k\neq \ell+a_\ell$ for all $1\le k<\ell$. \\\\ Prove that there exist two positive integers $b$ and $N$ for which\[\left\vert\sum_{j=m+1}^n(a_j-b)\right\vert\le1007^2\]for all integers $m$ and $n$ such that $n>m\ge N$. \begin{flushright}\emph{(Australia)}\end{flushright}