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