DescriptionTime
Limit: 1sec Memory Limit:256MB
You can approximate PI by using the following series:
PI = 4(1 - 1/3 + 1/5 - 1/7 + 1/9 - ... -1/(2i-1) + 1/(2i+1))
Input
An integer i (0<i<=100000).
Output
The PI value.
You should set the precision to 15.
Sample Input
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10
Sample Output
3.232315809405593
一开始老是超时，最后放弃了按顺序叠加的方法（因为那要频繁计算一个pow(-1,n)），而是把加法看成两段，奇偶分开算，就没超时了。