#include <iostream>
#include <cmath>
using namespace std;

double f(double x)
{
	return(0.25*(1 - 2 * pow(x, 3)));
}

int main()
{
	setlocale(LC_ALL, "Russian");
	double x = 0, x1 = 11, e;
	cout << "e: ";
	cin >> e;
	while (fabs(x1 - x) > e)
	{
		x = x1;
		x1 = f(x);
	}
	cout << x1 << endl;
}