#include <iostream>
#include <vector>
#include <memory>
#include <cmath>
#include <iomanip>
using namespace std;
template <typename T>
T sum(T e) {
	unsigned long long n = 1;
	T s = -1.5;
	T a = -1.5;
	while (abs(a) > e) {
		n++;
		a = pow(-1, n) * (((2*n) +1)/(pow(n,3)*(n+1)));
		s += a;
	};
	return  s;
}
int main() {
	double e = 0;
	cin >> e;
	if (e < 10e-7) {
		cout << fixed << setprecision(abs(log10(e))) << sum(float(e));
	}
	else {
		cout << fixed << setprecision(abs(log10(e))) << sum(double(e));
	}
	return 0;
}