#include <bits/stdc++.h>
using namespace std;
int n;
int main() {
  cin >> n;
  cout << n + n / 2 << 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;
}
