#include <iostream>

using namespace std;

int N;

int main() {
	cin >> N;
	int e2 = N * 7;
	if (e2 % 2 == 0) {
		cout << e2 / 2 << endl;
	} else {
		cout << e2 / 2 << ".5" << endl;
	}
	return 0;
}