1/1 + 1/2 + 1/3 + 1/4 + ... 在数学上称为调和级数。
　它是发散的，也就是说，只要加上足够多的项，就可以得到任意大的数字。
　但是，它发散的很慢：
　前1项和达到 1.0
　前4项和才超过 2.0
　前83项的和才超过 5.0
　那么，请你计算一下，要加多少项，才能使得和达到或超过 15.0 呢？(1/1 +, 1/2 +, 1/3 +, 1/4 +... Are mathematically called harmonic series.
It is divergent, that is, as long as you add enough items, you can get any large number.
But it diverges very slowly:
The first 1 and reach 1
First 4 and only over 2
The sum of the first 83 is only over 5
Well, please count it. How many items can you add to make it reach or exceed 15?)