#include <bits/stdc++.h>
using namespace std;
signed main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  long long n;
  cin >> n;
  cout << n / 2 + n << "\n";
  for (long long i = 2; i <= n; i += 2) cout << i << " ";
  for (long long i = 1; i <= n; i += 2) cout << i << " ";
  for (long long i = 2; i <= n; i += 2) cout << i << " ";
  return 0;
}
