#include <bits/stdc++.h>
using namespace std;
int main() {
  ios::sync_with_stdio(false);
  vector<int> v;
  int n;
  cin >> n;
  for (int i = 2; i <= n; i += 2) v.push_back(i);
  for (int i = 1; i <= n; i += 2) v.push_back(i);
  for (int i = 2; i <= n; i += 2) v.push_back(i);
  cout << v.size() << endl;
  for (auto p : v) cout << p << " ";
  cout << endl;
}
