5.3.15

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

using namespace std;

int main()
{
    double s, eps, a;
    s = 0;
    a = 1;
    cin >> eps;
    while (a >= eps) {
        a /= 2;
        s += a;
    }
    cout << s << endl;
    return 0;
}