#include <iostream>
#include <vector>
#include <algorithm>
#include <string>
#include <iomanip>
#include <functional>
#include <cmath>
using namespace std;
#define lli long long int

int main() {
	int n;
	lli x = 0;
	cin >> n;
	cout << fixed << setprecision(1) << 3.5 * n << endl;
	return 0;
}