If a natural number can be expressed as the square difference of two non-zero natural numbers, then this number is called "auspicious number", such as: 9=5**2-4**2, 9 is "auspicious number". So what is the 2013th "auspicious number" among the natural numbers starting from 1?
如果一个自然数能表示成两个非零自然数的平方差，则称这个数为"吉祥数"，如：9=5**2-4**2，9是"吉祥数"．那么从1开始的自然数中，第2013个"吉祥数"是多少．
<result>2_687</result>