#include <bits/stdc++.h>
using namespace std;
const long long int INF = 10e7;
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  long long int n, i, f = 0, x;
  cin >> n;
  vector<long long int> v(n + 1), ans;
  set<long long int> s1, s2;
  for (i = 0; i < n; i++) {
    cin >> v[i];
    if (v[i] > i + 1) f = 1;
    x = v[i];
    s1.insert(x);
  }
  v[n] = INF;
  if (f == 1) {
    cout << "-1\n";
    return 0;
  }
  long long int mex = 0;
  for (i = 0; i < n; i++) {
    while (s1.find(mex) != s1.end()) mex++;
    if (s2.empty() == true) {
      ans.push_back(mex);
    } else {
      auto it = s2.begin();
      ans.push_back(*it);
      s2.erase(it);
    }
    if (v[i] != v[i + 1]) {
      auto it = s1.find(v[i]);
      s2.insert(*it);
    }
    if (s2.empty() == true) mex++;
  }
  for (auto it : ans) cout << it << " ";
  return 0;
}
