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