#include <iostream>

using namespace std;

int n;

int main() {
    cin >> n;
    long long ans;
    if (n % 2 == 1) {
        ans = 3 * (n / 2) + 1;
    } else {
        ans = 3 * (n / 2 - 1) + 2;
    }
    cout << 3 * ans << "\n";
    return 0;
}