#include <iostream>
#include <cmath>
#include <algorithm>
#include <string>
#include <vector>
#include <map>
#include <unordered_map>
#include <queue>
#include <set>
#include <unordered_set>
#include <complex>
#include <list>
#include <cassert>
#include <chrono>
#include <random>
#include <stack>
#include <iomanip>
#include <fstream>
using namespace std;
#define endl "\n"
// #define int long long
const int INF = 2 * 1e5 + 5;
const int MOD = 1e9 + 7;
int mex(int n, vector<int> a){
    sort(a.begin(), a.end());
    int k = 1;
    for(int i = 0; i < n; i++){
        if(a[i] != k){
            return k;
        } else {
            k++;
        }
    }
    return k+1;
}
void solve(){
    int n;
    cin >> n;
    vector<int> b(n);
    for(int i = 0; i < n; i++){
        cin >> b[i];
    }
    int m = 2*n-1;
    vector<int> sol;
    sol.push_back(b[0]);
    int l = mex(sol.size(), sol), r = m;
    for(int i = 1; i < n; i++){
        if(b[i] == b[i-1]){
            sol.push_back(l);
            sol.push_back(r);
        } else {
            sol.push_back(b[i]);
            sol.push_back(r);
        }
        l = mex(sol.size(), sol);
        r--;
    }
    for(auto &i : sol){
        cout << i << " ";
    }
    cout << endl;
}
int32_t main(){
    ios_base::sync_with_stdio(false); cin.tie(nullptr);
    int t = 1;
    // cin >> t;
    while(t--) solve();
    return 0;
}
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