#include <bits/stdc++.h>
using namespace std;
const int INF = 2e9 + 5;
int main() {
  cin.tie(0);
  cin.sync_with_stdio(0);
  int n;
  cin >> n;
  vector<int> v(n, 2);
  cout << n + n / 2 << '\n';
  for (int i = 1; i < n; i += 2) {
    cout << i + 1 << ' ';
    v[i] = 1;
  }
  for (int i = 0; i < n; i += 2) {
    cout << i + 1 << ' ';
    v[i] = 0;
  }
  for (int i = 1; i < n; i += 2) {
    cout << i + 1 << ' ';
    v[i] = 0;
  }
}
