#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  while (~scanf("%d", &n)) {
    printf("%d\n", n + n / 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);
    printf("\n");
  }
  return 0;
}
