#include <bits/stdc++.h>
using namespace std;
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
void c_p_c() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
}
long long int vis[100005];
long long int arr[100005];
long long int ans[100005];
int32_t main() {
  c_p_c();
  long long int n;
  cin >> n;
  long long int f = 0;
  for (long long int i = 1; i <= n; i++) {
    cin >> arr[i];
    if (arr[i] > i) {
      f = 1;
    }
    vis[arr[i]] = 1;
  }
  if (f) {
    cout << -1 << "\n";
    return 0;
  }
  memset(ans, -1, sizeof(ans));
  for (long long int i = 2; i <= n; i++) {
    if (arr[i] != arr[i - 1]) {
      ans[i] = arr[i - 1];
      vis[arr[i - 1]] = 1;
    }
  }
  vector<long long int> not_vis;
  for (long long int i = 0; i <= 100004; i++) {
    if (vis[i] != 1) {
      not_vis.push_back(i);
    }
  }
  long long int j = 0;
  for (long long int i = 1; i <= n; i++) {
    if (ans[i] == -1) {
      ans[i] = not_vis[j++];
    }
  }
  for (long long int i = 1; i <= n; i++) {
    cout << ans[i] << " ";
  }
  cout << "\n";
  return 0;
}
