#include <bits/stdc++.h>
using namespace std;
int max(int a, int b, int c) { return max(a, max(b, c)); }
const int MAX = 5001;
const int INF = 1e6;
int main() {
  ios::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  int n;
  cin >> n;
  cout << n + n / 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 << " ";
  return 0;
}
