#include <bits/stdc++.h>
using namespace std;
inline int getn() {
  int sign = 1, num;
  char c;
  c = getchar();
  for (; c < '0' || c > '9'; c = getchar())
    if (c == '-') sign = -1;
  for (num = 0; (c >= '0' && c <= '9'); c = getchar()) num = num * 10 + c - '0';
  return num * sign;
}
int main() {
  int n;
  scanf("%d", &n);
  printf("%d", n / 2 * 3);
}
