#include <bits/stdc++.h>
const int maxN = 3e5 + 5;
int main() {
  if (fopen("iput.txt", "r")) {
    freopen("iput.txt", "r", stdin);
    freopen("oput.txt", "w", stdout);
  }
  int n;
  scanf("%d", &n);
  printf("%d\n", n / 2 * 2 + n - n / 2);
  printf("2 ");
  for (int i = 4; i <= n; i += 2) printf("%d ", i);
  for (int i = 1; i <= n; i += 2) printf("%d ", i);
  for (int i = 2; i <= n; i += 2) printf("%d ", i);
  return 0;
}
