#include <bits/stdc++.h>
using namespace std;
int n, m, i, j;
int main() {
  ios::sync_with_stdio(false);
  cin >> n;
  cout << 3 * n / 2 << '\n';
  for (i = 2; i <= n; i += 2) cout << i << ' ';
  for (i = 1; i <= n; i += 2) cout << i << ' ';
  for (i = 2; i <= n; i += 2) cout << i << ' ';
  return 0;
}
