#include <bits/stdc++.h>
using namespace std;
int main() {
  long long int i, j, k, n, a, b, c, m, x, y, z, num,
      count = 0, count1 = 0, count2 = 0, sum = 0, sum1 = 0, sum2 = 0, check = 0,
      len = 0, small = 0, big = 0;
  string str, str1, str2;
  cin >> n;
  cout << 3 * (n / 2);
  return 0;
}
