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