#include <iostream>
#include <algorithm>
#include <cstring>
#include <cmath>
#include <cstdlib>
#include <vector>
#include <map>
#include <set>
#include <string>
using namespace std;
long long k = 3;
long long n;
int main()
{
    cin.sync_with_stdio(false);
    cout.sync_with_stdio(false);
    cin >> n;
    if (n % 2 == 0)
    {
        k = k + (n / 2LL) * 9LL - 6LL;
    }
    else
    {
        k = k + (n / 2LL) * 9LL;
    }
    cout << k;



    return 0;
}