#include <stdio.h>
#include <bits/stdc++.h>

using namespace std;

#define ll long long
#define ld long double
#define pb push_back
#define mp make_pair
#define pii pair<int, int>
#define pll pair<ll, ll>
#define pdd pair<ld, ld>
#define all(x) (x).begin(), (x).end()
#define fi first
#define se second

int main() {
  cin.sync_with_stdio(false);

  ll n;
  cin >> n;

  ll mlc = (n + 2) / 2;
  mlc *= 3;

  ll mlc2 = (n - 1) / 2;
  mlc2 *= 3;
  cout << mlc + 2LL * mlc2 << '\n';

  return 0;
}