#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  if (n <= 1) {
    cout << "1 1";
  } else {
    int m = n / 2 + (n - n / 2) + n / 2;
    cout << m << 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 << " ";
  }
  return 0;
}
