6. (1988 Mathematics Retest Question for Science Experimental Class Admission) Let $S=[\sqrt{1}]+[\sqrt{2}]+\cdots+[\sqrt{1988}]$, find $[\sqrt{S}]$.

保留源文本的换行和格式，翻译结果如下：

6. (1988 Mathematics Retest Question for Science Experimental Class Admission) Let $S=[\sqrt{1}]+[\sqrt{2}]+\cdots+[\sqrt{1988}]$, find $[\sqrt{S}]$.