#include <bits/stdc++.h>   using namespace std;   vector<long long> pref_sum_a;   vector<long long> pref_sum_b;   signed main() {     long long n;     cin >> n;     if (n == 2) {       cout << 3 << endl;       cout << 2 <<     << 1 <<     << 2 << endl;     }     if (n == 3) {       cout << 4 << endl;       cout << 2 <<     << 1 <<     << 3 <<     << 2 << endl;     }     if (n == 4) {       cout << 6 << endl;       cout << 1 <<     << 3 <<     << 2 <<     << 4 <<     << 1 <<     << 3            << endl;     }     if (n > 4) {       cout << n + n / 2 << endl;       for (long long t = 2; t <= n; t += 2) {         cout << t <<    ;       }       for (long long g = 1; g <= n; g += 2) {         cout << g <<    ;       }       for (long long t = 2; t <= n; t += 2) {         cout << t <<    ;       }     }   }   