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