#include <bits/stdc++.h>
using namespace std;
queue<int> Q;
int n;
int main() {
  cin >> n;
  for (int i = 2; i <= n; i++)
    if (i % 2 == 0) Q.push(i);
  for (int i = 1; i <= n; i++)
    if (i % 2 == 1) Q.push(i);
  for (int i = 2; i <= n; i++)
    if (i % 2 == 0) Q.push(i);
  cout << Q.size() << endl;
  while (!Q.empty()) {
    printf("%d", Q.front());
    Q.pop();
    if (!Q.empty())
      printf(" ");
    else
      printf("\n");
  }
  return 0;
}
