#include <bits/stdc++.h>
const int mx = 1e5 + 10;
using namespace std;
int a[mx], b[mx];
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  int n, ans = 0;
  cin >> n;
  ans = n + n / 2;
  cout << ans << 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 << " ";
}
