#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  scanf("%d", &n);
  ;
  vector<int> res;
  for (int x = 2; x <= n; x += 2) res.emplace_back(x);
  for (int x = 1; x <= n; x += 2) res.emplace_back(x);
  for (int x = 2; x <= n; x += 2) res.emplace_back(x);
  printf("%d\n", (int)res.size());
  for (auto it : res) printf("%d ", it);
}
