#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  printf("%d\n2", n + n / 2);
  for (long long i = 4; i <= n; i++) printf(" %d", i++);
  for (long long i = 1; i <= n; i++) printf(" %d", i++);
  for (long long i = 2; i <= n; i++) printf(" %d", i++);
  printf("\n");
}
