#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  scanf("%d", &n);
  printf("%d\n", (n * 3) / 2);
  for (int i = 2; 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;
}
