Let $$S^{}_{}$$ be a subset of $$\{1,2,3^{}_{},\ldots,1989\}$$ such that no two members of $$S^{}_{}$$ differ by $$4^{}_{}$$ or $$7^{}_{}$$. What is the largest number of elements $$S^{}_{}$$ can have?

(第七届AIME1989 第13题)