#include <bits/stdc++.h>
#pragma comment(linker, "/stack:200000000")
#pragma GCC optimize("Ofast")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
using namespace std;
char alpha[26] = {'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I',
                  'J', 'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R',
                  'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z'};
int main() {
  int n;
  cin >> n;
  cout << n + (n / 2) << "\n";
  for (int i = 2; i <= n; i = i + 2) {
    cout << i << " ";
  }
  for (int i = 1; i <= n; i = i + 2) {
    cout << i << " ";
  }
  for (int i = 2; i <= n; i = i + 2) {
    cout << i << " ";
  }
  return 0;
}
