#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  long long int q;
  long long int i, j, k, sum = 0, a, b, c, d, e = -1, n, count = 0, count1 = 0,
                         sum1 = 0, l, h, uh;
  cin >> n;
  cout << (n / 2) * 3 << '\n';
  return 0;
}
