#include <iostream>
using namespace std;

double K;

double E(double x) {
	if (x >= K) return 0;
	return (E(x + 1) + E(x + 2) + E(x + 3) + E(x + 4) + E(x + 5) + E(x + 6)) / 6 + 1;
}

int main() {
	double x = 0;
	cin >> K;
	cout << E(x);
}