#include <bits/stdc++.h>
#define PI 3.14159265359
using namespace std;
const int64_t MOD = 1e9 + 7;

int main() {
	int N;
	cin >> N;

	double n = 21.0 / 6.0 * N;

	cout << fixed << setprecision(3) << n << endl;
}