Let $S$ be the set of ordered integer pairs $(x, y)$ such that $0 < x < y < 42$ and there exists some integer $n$ such that $x^6-y^6 \mid n^2+2015^2$. What is the sum $\sum_{(x_i, y_i) \in S}x_iy_i$?


Let's think step by step and output the final answer within \boxed{}.
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