#include <bits/stdc++.h>
using namespace std;
int n;
int main() {
  scanf("%d", &n);
  if (n == 2) {
    cout << 3 << endl;
    cout << "2 1 2";
    return 0;
  }
  cout << n + n / 2 << endl;
  for (int i = 1; i <= n; i++) {
    if (i % 2 == 0) cout << i << " ";
  }
  for (int i = 1; i <= n; i++) {
    if (i % 2 == 1) cout << i << " ";
  }
  for (int i = 1; i <= n; i++) {
    if (i % 2 == 0) cout << i << " ";
  }
}
