#include <bits/stdc++.h>
using namespace std;
const long long N = 2 * 1e5 + 4;
long long add(long long a, long long b, long long m) {
  if (a < 0) a += m;
  if (b < 0) b += m;
  long long res = ((a % m) + (b % m)) % m;
  res %= m;
  return res;
}
long long mul(long long a, long long b, long long m) {
  if (a < 0) a += m;
  if (b < 0) b += m;
  long long res = ((a % m) * (b % m)) % m;
  res %= m;
  return res;
}
long long sub(long long a, long long b, long long m) {
  long long res = (a - b + m) % m;
  res %= m;
  return res;
}
int32_t main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  long long n;
  cin >> n;
  vector<long long> res(n, -1);
  set<long long> second;
  long long prev = 0;
  for (long long i = 0; i < n; i++) {
    long long cur;
    cin >> cur;
    second.insert(cur);
    if (cur != prev) res[i] = prev;
    prev = cur;
  }
  second.insert(0);
  long long idx = 0;
  for (long long i = 0; i < n; i++) {
    if (res[i] != -1) continue;
    while (second.find(idx) != second.end()) {
      idx++;
      cerr << idx << '\n';
    }
    second.insert(idx);
    res[i] = idx;
  }
  for (auto x : res) cout << x << " ";
  return 0;
}
