#include <bits/stdc++.h>
using namespace std;
int res[330000];
int main() {
  int ans, n, cont = 0;
  scanf("%d", &n);
  ans = n / 2 * 3;
  if (n & 1) ans++;
  printf("%d\n", ans);
  for (int i = 2; i <= n; i += 2) res[cont++] = i;
  for (int i = 1; i <= n; i += 2) res[cont++] = i;
  for (int i = 2; i <= n; i += 2) res[cont++] = i;
  for (int i = 0; i < cont; i++) {
    printf("%d", res[i]);
    if (i == cont - 1)
      puts("");
    else
      printf(" ");
  }
  return 0;
}
