#include <bits/stdc++.h>
using namespace std;
const long long mod = 1000000007;
const long long MAXX = 1e6 + 5;
long long t, n, x;
long long a[MAXX];
signed main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  cin >> n;
  for (long long i = 0; i < n; i++) {
    cin >> a[i];
  }
  if (n == 1) {
    if (a[0] == 0) {
      cout << "1" << endl;
      return 0;
    }
    cout << "0" << endl;
    return 0;
  }
  vector<long long> pos(n + 2, 0);
  vector<long long> b(n + 2, 0);
  for (long long i = 0; i <= n; i++) {
    b[i] = i;
    pos[i] = i;
  }
  long long y = 0;
  for (long long i = 1; i < n; i++) {
    if (a[i] != a[i - 1]) {
      swap(b[pos[a[i - 1]]], b[i]);
      swap(pos[a[i - 1]], pos[i]);
      continue;
    }
  }
  long long i = n;
  swap(b[pos[a[i - 1]]], b[i]);
  swap(pos[a[i - 1]], pos[i]);
  bool f = true;
  long long x = 0;
  vector<bool> used(MAXX, false);
  for (long long i = 0; i < n; i++) {
    used[b[i]] = true;
    while (used[x]) {
      x++;
    }
    if (a[i] != x) {
      f = false;
      break;
    }
  }
  if (!f) {
    cout << "-1" << endl;
    return 0;
  }
  for (long long i = 0; i < n; i++) {
    cout << b[i] << " ";
  }
  return 0;
}
