#include <bits/stdc++.h>
using namespace std;
vector<int> vec1, vec2;
int main() {
  int n;
  scanf("%d", &n);
  for (int i = 1; i <= n; i++) {
    if (i % 2)
      vec1.push_back(i);
    else
      vec2.push_back(i);
  }
  int m = (vec1.size() + vec2.size() * 2);
  printf("%d\n", m);
  for (int i = 0; i < vec2.size(); i++) printf("%d ", vec2[i]);
  for (int i = 0; i < vec1.size(); i++) printf("%d ", vec1[i]);
  for (int i = 0; i < vec2.size(); i++) printf("%d ", vec2[i]);
}
