#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  if (n & 1)
    cout << ((n + 1) >> 1) + (n - 1) << endl;
  else
    cout << n + (n / 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 << " ";
}
