#include <bits/stdc++.h>
using namespace std;
#pragma GCC optimize("Ofast")
template <typename T, typename U>
inline bool exist(T &cont, U &val) {
  return cont.find(val) != cont.end();
}
struct custom_hash {
  static uint64_t splitmix64(uint64_t x) {
    x += 0x9e3779b97f4a7c15;
    x = (x ^ (x >> 30)) * 0xbf58476d1ce4e5b9;
    x = (x ^ (x >> 27)) * 0x94d049bb133111eb;
    return x ^ (x >> 31);
  }
  size_t operator()(uint64_t x) const {
    static const uint64_t FIXED_RANDOM =
        chrono::steady_clock::now().time_since_epoch().count();
    return splitmix64(x + FIXED_RANDOM);
  }
};
int main(int argc, char **argv) {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  int n;
  cin >> n;
  vector<int> v(n);
  set<int> s1;
  for (int i = 0, _n = (n); i < _n; i++) {
    cin >> v[i];
    s1.insert(v[i]);
    if (v[i] > i + 1) {
      cout << -1 << '\n';
      return 0;
    }
  }
  vector<int> ans(n, -1);
  vector<int> ranges;
  for (int i = (1), _b = (n - 1); i <= _b; i++) {
    if (v[i - 1] != v[i]) {
      ans[i] = v[i - 1];
    }
  }
  int start = !v[0];
  for (int i = 0, _n = (n); i < _n; i++) {
    if (ans[i] == -1) {
      while (s1.find(start) != s1.end()) {
        start++;
      }
      ans[i] = start++;
    }
  }
  for (int i = 0, _n = (n); i < _n; i++) {
    cout << ans[i] << ' ';
  }
  return 0;
}
