#include <bits/stdc++.h>
using namespace std;
int n;
int main() {
  cin >> n;
  int u = 0, m = 0;
  cout << n + n / 2 << endl;
  while (1) {
    if (u) {
      for (int i = 1; i <= n; i += 2) {
        cout << i << " ";
        m++;
      }
    } else {
      for (int i = 2; i <= n; i += 2) {
        cout << i << " ";
        m++;
      }
    }
    if (m == n + n / 2) return 0;
    u = !u;
  }
  return 0;
}
