#include <bits/stdc++.h>   using namespace std;   int main() {     int n;     cin >> n;     cout << n / 2 * 2 + (n + 1) / 2 << 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 <<    ;   }   