#include "bits/stdc++.h"

using namespace std;

using ll = long long;
using ld = long double;

const double PI = 3.1415926535897932384626433832795;
const ll MOD = 1000000007;

const int dx[] = { 0, 1, 0, -1 };
const int dy[] = { -1, 0, 1, 0 };

int gcd(int x, int y) { return y ? gcd(y, x % y) : abs(x); }
ll gcd(ll x, ll y) { return y ? gcd(y, x % y) : abs(x); }
int lcm(int x, int y) { return x / gcd(x, y) * y; }
ll lcm(ll x, ll y) { return x / gcd(x, y) * y; }

int main() {
	ios::sync_with_stdio(false);
	cin.tie(0);

	ll n;
	cin >> n;

	ll ans = 0;
	if (n % 2LL == 0LL) {
		ans = 3LL*(n / 2LL);
	} else {
		ans = 3LL*((n + 1) / 2LL) - 2;
	}

	cout << ans << endl;
	return 0;
}