/*
X = {1,2,3}
S = 231213
X = {1,2,3,4}
S = 23421314
 
최대한 왼쪽에 새로운 숫자를 넣는 방법을 선택
X의 크기는 8이하
X의 원소는 F이하
불가능한 경우도 있음 dfs탐색?
 */
#include <algorithm>
#include <iostream>
#include <vector>
using namespace std;
int arr[33], N, T;
void print(int *list) {
    for (int i = 1; i <= 2 * N; i++) {
        cout << list[i] << ' ';
    }
    cout << '\n';
}
bool dfs(vector<int> availables) {
    // ex. N=4
    //  __4____4
    if (availables.empty()) {
        for (int i = 1; i <= N; i++) {
            if (arr[i] == -1) {
                return false;
            }
        }
        return true;
    }
    int n = availables.at(availables.size() - 1);
    for (int i = 2 * N; i - n - 1 > 0; i--) {
        if (arr[i] == -1 && arr[i - n - 1] == -1) {
            arr[i] = arr[i - n - 1] = n;
            availables.pop_back();
            if (dfs(availables)) {
                return true;
            }
            availables.emplace_back(n);
            arr[i] = arr[i - n - 1] = -1;
        }
    }
 
    return false;
}
 
int main() {
    cin >> N;
    for (int i = 1; i <= 2 * N; i++) {
        arr[i] = -1;
    }
    vector<int> X;
    for (int i = 0; i < N; i++) {
        cin >> T;
        X.emplace_back(T);
    }
    sort(X.begin(), X.end());
 
    int n = X.at(X.size() - 1);
    arr[2*N-n-1] = arr[2 * N] = n;
    X.pop_back();
    if (dfs(X)) {
        print(arr);
        return 0;
    }
    cout << -1;
}
 

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7
1 2 3 4 5 6 7