#include <bits/stdc++.h>
#pragma comment(linker, "/STACK:256000000")
const long long INF = INT_MAX;
const long long mod = 998244353;
const long long LINF = 1e14;
using namespace std;
int main() {
  std::ios::sync_with_stdio(false);
  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;
}
