#include <bits/stdc++.h>
using namespace std;
int main() {
  long long n;
  cin >> n;
  long long ans = n / 2 + n / 2 + (n + 1) / 2;
  cout << ans << "\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;
}
