#include <bits/stdc++.h>
#define ll long long
using namespace std;
set<ll> s;
ll l,r;
ll Get_First()
{
while (s.count(l))
l++;
s.insert(l);
return l;
}
ll Get_Last()
{
while (s.count(r))
r--;
s.insert(r);
return r;
}
int main()
{
ios::sync_with_stdio(false); cin.tie(0); cout.tie(0);
ll n,b;
cin >> n;
l=1;
r=2*n-1;
cin >> b;
cout << b << " ";
s.insert(b);
ll prev=b;
for (ll i=2;i<=n;i++)
{
cin >> b;
if (b==prev)
cout << Get_First() << " " << Get_Last() << " ";
else if (!s.count(b))
{
cout << b << " ";
s.insert(b);
if (prev<b)
cout << Get_Last() << " ";
else
cout << Get_First() << " ";
}
else
{
if (prev>b)
cout << Get_First() << " " << Get_First() << " ";
else
cout << Get_Last() << " " << Get_Last() << " ";
}
prev=b;
}
return 0;
}
