#include <bits/stdc++.h>
using namespace std;
using ll = long long;
const int mod = 1e9 + 7;
const int inf = (1 << 30) - 1;
const ll infll = (1LL << 61) - 1;

int N, ans;

int main()
{
    cin >> N;
    ans = 3 * (N / 2) + (N % 2);
    cout << ans << endl;
}