#include <bits/stdc++.h>
using namespace std;
bool myAssert(bool b);
void testDrivenDevelopment();
int start(int argc = 0, char const *argv[] = NULL);
bool *isvisited;
int main(int argc, char const *argv[]) {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  int n;
  cin >> n;
  n = n >> 1;
  n = 3 * n;
  cout << n << "\n";
  return 0;
}
