#include<bits/stdc++.h> using namespace std; typedef long long ll; int main() { ll n; cin >> n; vector<ll> b(n+5); for(ll a = 1; a <= n; a++) { cin >> b[a]; } vector<ll> a; vector<ll> is_true((2*n)); for(ll i = 1; i <= n; i++) { if(is_true[b[i]] == 0) { is_true[b[i]] = 1; a.push_back(b[i]); } } for(ll i = 1; i < 2*n; i++) { if(is_true[i] == 0) { a.push_back(i); } } for(ll i = 0; i < 2*n-1; i++) { cout << a[i] << " "; } cout << endl; }