#include <iostream>
#include <vector>
using namespace std;
int n, b[100005];
bool mark[2 * 100005];
vector<int> ans;
int main() {
ios_base::sync_with_stdio(false);
cin.tie(NULL);
cin >> n;
for (int i = 0; i < n; i++) {
cin >> b[i];
}
mark[0] = mark[2 * n] = 1;
int left = 0, right = 2 * n;
ans.push_back(b[0]);
mark[b[0]] = 1;
for (int i = 1; i < n; i++) {
int added = 0;
if (not mark[b[i]]) {
ans.push_back(b[i]);
mark[b[i]] = 1;
added++;
}
while (mark[left]) left++;
while (mark[right]) right--;
if (added == 0) {
ans.push_back(left), ans.push_back(right);
mark[left] = mark[right] = 1;
} else {
if (b[i - 1] < b[i]) {
ans.push_back(right);
mark[right] = 1;
} else if (b[i - 1] > b[i]) {
ans.push_back(left);
mark[left] = 1;
}
}
}
for (int i : ans) {
cout << i << ' ';
}
cout << endl;
return 0;
}
