#include<bits/stdc++.h>
using namespace std;
using ll = long long;
const ll N = 2e5 + 2;
set < ll > uragsh_ind[N], aragsh_ind[N];
ll uragsh[N], aragsh[N], a[N], b[N], used[N] = {0};
int main() {
ll n, m, r, x,s, y, i, j,ind, ans, t;
cin >> n;
for (i = 1; i <= n; i ++) {
cin >> a[i];
}
for (i = 1; i <= 2 * n - 1; i ++) b[i]= -1;
b[1] = a[1];
used[a[1]] = 1;
for (i = 2; i <= n; i ++) {
if ( a[i] == a[i - 1]) {
uragsh[a[i]] = aragsh[a[i]] = 1;
uragsh_ind[a[i]].insert(i);
aragsh_ind[a[i]].insert(-i);
}
else {
if ( a[i] > a[i- 1]) {
aragsh_ind[a[i]].insert(-i);
aragsh[a[i]] =1;
if( used[a[i]] == 0) b[2 * i - 1] = a[i];
}
else {
uragsh_ind[a[i]].insert(i);
uragsh[a[i]] = 1;
if ( used[a[i]] == 0) b[2 * i - 1] = a[i];
}
used[a[i]] = 1;
}
}
queue < ll > q;
for (i =1 ; i <= 2 *n - 1; i ++) {
if ( used[i] == 1) {
for ( ll ind : uragsh_ind[i]) {
s = q.front();
q.pop();
b[ind * 2 - 2] = s;
}
}
else {
q.push(i);
}
}
stack < ll > v;
while(!q.empty()) {
v.push(q.front());
q.pop();
}
for (i = 2 * n - 1; i >= 1; i --) {
if ( used[i] == 1) {
for ( ll ind : aragsh_ind[i]) {
s = v.top();
v.pop();
ind =-ind;
if ( b[ind * 2 - 1] == -1) b[ind * 2 - 1] = s;
else b[ind * 2 - 2] = s;
}
}
}
for (i = 1; i <= 2 *n - 1; i ++) {
cout << b[i] << " ";
}
}
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