#include <bits/stdc++.h>
using namespace std;
int main() {
  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 << " ";
  cout << endl;
}
