#include <bits/stdc++.h>
using namespace std;
void solve() {
  int n;
  cin >> n;
  vector<int> in(n);
  for (auto& it : in) cin >> it;
  set<int> unk;
  int cur = 0;
  vector<int> res(n, -1);
  for (int i = 0; i < n; i++) {
    unk.insert(i);
    while (cur < in[i]) {
      if (unk.size() == 0) {
        cout << -1 << endl;
        return;
      }
      res[*unk.rbegin()] = cur;
      cur++;
      unk.erase(*unk.rbegin());
    }
  }
  set<int> all;
  for (int i = 0; i <= n; i++) {
    all.insert(i);
  }
  for (int j = 0; j < n; j++) {
    int i = res[j];
    all.erase(i);
    if (all.empty()) continue;
    if (*all.begin() != in[j]) {
      cout << -1;
      return;
    }
  }
  for (int i : res) {
    if (i == -1) i = n + 1;
    cout << i << " ";
  }
}
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  int t = 1;
  while (t--) {
    solve();
  }
}
