#include <iostream>
#include <cmath>
using namespace std;
int main() {
	float sum = 0, e, next = 1 / sqrt(3);
	cin >> e;
	while (next >= e) {
		sum += next;
		next *= 1/sqrt(3);
	}
	cout << "S = " << sum << "\n";
	system("pause");
	return 0;
}