#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  cout << n / 2 * 2 + (n + 1) / 2 << endl;
  for (int q = 2; q <= n; q += 2) {
    cout << q << " ";
  }
  for (int q = 1; q <= n; q += 2) {
    cout << q << " ";
  }
  for (int q = 2; q <= n; q += 2) {
    cout << q << " ";
  }
  cout << endl;
}
