#include <iostream>

using namespace std;
const int N = 1e8;

int cnt, tmp;

int main()
{
	double res = 0;
	
	for (int i = 1; i < N; i += 2 )
	{
		cnt ++ % 2 ? tmp = -i : tmp = i;
		res += (double)1 / tmp;
	}
	printf("%.6f", res * 4);
	return 0;
}