#include <bits/stdc++.h>
using namespace std;
using pii = pair<int, int>;
using ll = long long;
#define NMAX 100010


int main()
{
	#ifndef ONLINE_JUDGE
	freopen("data.in", "r", stdin);
	freopen("data.out", "w", stdout);
	#endif

	int i, n;

	cin >> n;

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

	return 0;
}