#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  int n;
  cin >> n;
  cout << n + (n / 2) << '\n';
  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 << ' ';
  cout << '\n';
}
