#include <bits/stdc++.h>
using namespace std;
int n;
int main() {
  cin >> n;
  cout << n + n / 2 << '\n';
  for (int i = 1; i <= n; i++)
    if (i % 2 == 0) cout << i << " ";
  for (int i = 1; i <= n; i++)
    if (i % 2 == 1) cout << i << " ";
  for (int i = 1; i <= n; i++)
    if (i % 2 == 0) cout << i << " ";
}
