sicily Computing PI

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。