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