#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  scanf("%d", &n);
  vector<int> vec;
  for (int i = 2; i <= n; i += 2) vec.push_back(i);
  for (int i = 1; i <= n; i += 2) vec.push_back(i);
  for (int i = 2; i <= n; i += 2) vec.push_back(i);
  printf("%d\n", vec.size());
  for (int i = 0; i < vec.size(); i++)
    printf("%d%c", vec[i], " \n"[i + 1 == vec.size()]);
  return 0;
}
