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