#include <bits/stdc++.h>
const int N = 1e4 + 5;
const int mod = 1e9 + 7;
const int MOD = mod - 1;
const double eps = 1e-18;
const double PI = acos(-1.0);
using namespace std;
char s[N];
int main() {
  int n;
  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%c", i, i + 2 <= n ? ' ' : '\n');
  }
  return 0;
}
