#include <bits/stdc++.h>
using namespace std;
int main() {
  std::ios::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  int n, m, i, j;
  cin >> n;
  cout << (n / 2 + ceil(n / 2.0) + n / 2) << "\n";
  for (i = 2; i <= n; i += 2) cout << i << " ";
  for (i = 1; i <= n; i += 2) cout << i << " ";
  for (i = 2; i <= n; i += 2) cout << i << " ";
  cout << "\n";
  return 0;
}
