#include <cstdlib>
#include <iostream>

using namespace std;

int main()
{
	double e;
	cin >> e;
	double a = 1.0, b = 25.0;
	double r=0, y=0;
	do{
		r+=y;
		y=1.0/(a+b);
		a *= 4;
		b *= 5;
	}while( y>e );
	cout << r << endl;
}