#include <bits/stdc++.h>
using namespace std;
long long ncrp(long long n, long long r, long long p) {
  long long C[r + 1];
  memset(C, 0, sizeof(C));
  C[0] = 1;
  for (long long i = 1; i <= n; i++)
    for (long long j = min(i, r); j >= 1; j--) C[j] = (C[j] + C[j - 1]) % p;
  return C[r];
}
long long mod_exp(long long x, long long y, long long mm) {
  x %= mm;
  if (y == 0)
    return (1);
  else if (y % 2 == 0)
    return (mod_exp((x * x) % mm, y / 2, mm));
  else
    return ((x * mod_exp((x * x) % mm, (y - 1) / 2, mm)) % mm);
}
long long node(long long i, long long j, long long m) { return m * i + j; }
pair<long long, long long> loc(long long z, long long m) {
  return {z / m, z % m};
}
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  long long n, r = 0, e;
  cin >> n;
  vector<long long> z;
  long long a[n], ans[n];
  for (long long i = 0; i < n; i++) {
    cin >> a[i];
    ans[i] = 1000000;
  }
  for (long long i = 0; i < n; i++) {
    if (r == a[i])
      z.push_back(i);
    else if (r == a[i] - 1) {
      ans[i] = r;
      r++;
    } else {
      z.push_back(i);
      e = z.size();
      for (long long j = e - 1; j >= 0; j--) {
        ans[z[z.size() - 1]] = r;
        r++;
        z.pop_back();
        if (r == a[i]) break;
        if (z.empty()) break;
      }
      if (r != a[i]) {
        cout << -1 << endl;
        return 0;
      }
    }
  }
  for (long long i = 0; i < n; i++) cout << ans[i] << " ";
  return 0;
}
