#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  if (n <= 3) {
    if (n == 2)
      cout << n / 2 << endl;
    else
      cout << n / 2 + 1 << endl;
    for (int i = 1; i <= n; i += 2) {
      cout << i << " ";
    }
  } else {
    cout << n << endl;
    for (int i = 2; i <= n; i += 2) {
      cout << i << " ";
    }
    for (int i = 1; i <= n; i += 2) {
      cout << i << " ";
    }
  }
  cout << endl;
  return 0;
}
