#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 lastMn, lastMx;
int mn(int m, vector<int> &visited){
for(int i = lastMn; i <= m; i++){
if(!visited[i]){
return i;
}
}
return m+1;
}
int mx(int m, vector<int> &visited){
for(int i = lastMx; i >= 1; i--){
if(!visited[i]){
return i;
}
}
return 0;
}
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(m), visited(m+1, 0);
sol[0] = b[0];
visited[b[0]] = 1;
lastMn = 1;
lastMx = m;
int l = mn(m, visited), r = mx(m, visited);
for(int i = 1; i < n; i++){
if(b[i] == b[i-1]){
l = mn(m, visited);
sol[2*i-1] = l;
visited[l] = 1;
r = mx(m, visited);
sol[2*i] = r;
visited[r] = 1;
} else if(b[i] > b[i-1]){
if(visited[b[i]]){
r = mx(m, visited);
visited[r] = 1;
sol[2*i-1] = r;
r = mx(m, visited);
visited[r] = 1;
sol[2*i] = r;
} else {
sol[2*i-1] = b[i];
visited[b[i]] = 1;
r = mx(m, visited);
sol[2*i] = r;
visited[r] = 1;
}
} else {
if(visited[b[i]]){
l = mn(m, visited);
visited[l] = 1;
sol[2*i-1] = l;
l = mn(m, visited);
sol[2*i] = l;
visited[l] = 1;
} else {
sol[2*i-1] = b[i];
visited[b[i]] = 1;
l = mn(m, visited);
sol[2*i] = l;
visited[l] = 1;
}
}
lastMn = l;
lastMx = 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;
}
