#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base ::sync_with_stdio(false), cin.tie(0), cout.tie(0);
  int n;
  cin >> n;
  cout << n + (n / 2) << endl;
  for (int i = 2; i <= n; i += 2) cout << i << ' ';
  for (int i = 1; i <= n; i += 2) cout << i << ' ';
  for (int i = 2; i <= n; i += 2) cout << i << ' ';
  return 0;
}
