#include <bits/stdc++.h>
using namespace std;
long long int power(long long int x, long long int y, long long int m) {
  if (y == 0) return 1;
  long long int p = power(x, y / 2, m) % m;
  p = (p * p) % m;
  return (y % 2 == 0) ? p : (x * p) % m;
}
long long int nCr(long long int n, long long int r, long long int m) {
  if (r > n) return 0;
  long long int a = 1, b = 1, i;
  for (i = 0; i < r; i++) {
    a = (a * n) % m;
    --n;
  }
  while (r) {
    b = (b * r) % m;
    --r;
  }
  return (a * power(b, m - 2, m)) % m;
}
void solve() {
  int n;
  cin >> n;
  vector<int> v(n);
  int f = 0;
  map<int, int> mp, mp2;
  for (int i = 0; i < n; ++i) {
    cin >> v[i];
    mp[v[i]] = 1;
    if (i != 0 && v[i - 1] > v[i]) f = 1;
  }
  if (f || v[0] > 1) {
    cout << -1 << '\n';
    return;
  }
  vector<int> ans(n);
  int l = 0;
  while (mp[l] == 1) ++l;
  if (v[0] == 1) {
    ans[0] = l;
    l++;
  } else {
    ans[0] = l;
    ++l;
  }
  while (mp[l] == 1) ++l;
  for (int i = 1; i < n; ++i) {
    while (mp[l] == 1) ++l;
    if (v[i] != v[i - 1])
      ans[i] = v[i - 1];
    else
      ans[i] = l++;
    while (mp[l] == 1) ++l;
  }
  int mex = 0;
  for (int i = 0; i < n; ++i) {
    mp2[ans[i]] = 1;
    while (mp2[mex] == 1) mex++;
    if (mex != v[i]) f = 1;
  }
  if (f) {
    cout << -1 << '\n';
    return;
  }
  for (int i = 0; i < n; ++i) cout << ans[i] << " ";
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  solve();
  return 0;
}
