#include <bits/stdc++.h>
using namespace std;
int n;
int main() {
  cin >> n;
  int ans = 0;
  if (n % 2 == 0) {
    ans = 3 * (n / 2);
  } else {
    ans = 3 * (n / 2) + 1;
  }
  cout << ans << endl;
  for (int i = 1; i <= n; i++) {
    if (!(i % 2)) cout << i << " ";
  }
  for (int i = 1; i <= n; i++) {
    if (i % 2) cout << i << " ";
  }
  for (int i = 1; i <= n; i++) {
    if (!(i % 2)) cout << i << " ";
  }
  return 0;
}
