#include <bits/stdc++.h>
using namespace std;
stringstream ss;
const int N = 3e5 + 5;
const int mod = 1e9 + 7;
int main() {
  ios_base::sync_with_stdio(false);
  int t = 1;
  while (t--) {
    int n;
    cin >> n;
    int tt = n / 2 + n / 2 + (n + 1) / 2;
    cout << tt << "\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 << " ";
  }
}
