#include <bits/stdc++.h>
using namespace std;
int main() {
  int i, j, k;
  int n;
  scanf("%d", &n);
  printf("%d\n", n + n / 2);
  for (i = 2; i <= n; i += 2) printf("%d ", i);
  for (i = 1; i <= n; i += 2) printf("%d ", i);
  for (i = 2; i <= n; i += 2) printf("%d ", i);
  printf("\n");
  return 0;
}
