#include <bits/stdc++.h>
using namespace std;
string numtostr(long long n) {
  ostringstream str1;
  str1 << n;
  return str1.str();
}
long long strtonum(string s) {
  long long x;
  stringstream str1(s);
  str1 >> x;
  return x;
}
long long GCD(long long a, long long b) {
  if (b == 0)
    return a;
  else
    return GCD(b, a % b);
}
long long LCM(long long a, long long b) {
  long long gcd = GCD(a, b);
  return (a * b) / gcd;
}
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  long long n;
  cin >> n;
  cout << n + n / 2 << endl;
  for (long long i = 1; i <= n; i++)
    if (i % 2 == 0) cout << i << " ";
  for (long long i = 1; i <= n; i++)
    if (i % 2 == 1) cout << i << " ";
  for (long long i = 1; i <= n; i++)
    if (i % 2 == 0) cout << i << " ";
  return 0;
}
