#include <bits/stdc++.h>
using namespace std;
const long long int MAXN = 1e5 + 10;
int a[MAXN];
int main() {
  int n;
  cin >> n;
  cout << n + (n / 2) << "\n";
  for (int i = 1; i < n; i += 2) cout << i + 1 << " ";
  for (int i = 0; i < n; i += 2) cout << i + 1 << " ";
  for (int i = 1; i < n; i += 2) cout << i + 1 << " ";
}
