#include <bits/stdc++.h>
using namespace std;
int dx[] = {0, 0, -1, 1, -1, 1, -1, 1};
int dy[] = {-1, 1, 0, 0, -1, -1, 1, 1};
int mod(int x, int y) { return ((x % y) + y) % y; }
int main() {
  int n;
  cin >> n;
  cout << (n / 2) * 3 << endl;
  return 0;
}
