#include <iostream>
#include <iomanip>
#include <algorithm>
#include <cmath>

using namespace std;

int main() {
	double eps;
	cin >> eps;
	double sum = 0, elem = 0.5;
	while (abs(elem) >= eps) {
		sum += elem;
		elem /= 2;
	}
	cout << fixed << setprecision(6);
	cout << sum + elem << endl;
	return 0;
}