#include <iostream>
#include <string>
#include <fstream>
#include <iomanip>
using namespace std;

int main()
{
	int n;
	long long a = 0;
	cin >> n;
	for (int i = 0; i < n; i++)
	{
		a += 7;
	}
	if (n % 2)cout << a / 2 << ".5" << endl;
	else cout << a / 2 << endl;
	return 0;
}