3.2.9

#include <iostream>
#include <cmath>

using namespace std;

int main()
{
    int n;
    double Sum=0;
    cin>>n;
    int k=0;
    for (int i=0;i<=n;i++)
    {
        if(i%2==0) k=1; else k=-1;
        Sum+=k*1.0/(2*i+1);
    }

    cout<<Sum*4;
    return 0;
}