``` class solution { int main() { int m, n; set<int> t_set; cin >> m; for (int i = 0,num;i < m;++i) { cin >> num; t_set.insert(num); } cin >> n; if (2 * n > t_set.size()) { cout << -1; return 0; } else { int sum = 0; auto itb = t_set.begin(); auto itrb = t_set.rbegin(); for (;--n > 0;itb++, itrb++) { sum += *itb; sum += *itrb; } cout << sum; return 0; } } }; ```