#include <bits/stdc++.h>
using namespace std;
template <typename T>
void printvec(vector<T>& vec) {
  for (int i = 0; i < vec.size(); i++) {
    cout << vec[i] << " ";
  }
  cout << endl;
}
int main() {
  ios_base::sync_with_stdio(false);
  long long int n;
  cin >> n;
  vector<long long int> inp(n);
  for (int i = 0; i < n; i++) {
    cin >> inp[i];
  }
  vector<long long int> ans(n, -1);
  vector<long long int> sp;
  long long int mex = 0;
  for (int i = 0; i < n; i++) {
    if (i != 0) {
      if (inp[i] < inp[i - 1]) {
        cout << "-1\n";
        return 0;
      }
    }
    if (mex != inp[i]) {
      ans[i] = mex;
      mex++;
      while (mex != inp[i]) {
        if (sp.size() == 0) {
          cout << "-1\n";
          return 0;
        }
        int ind = sp.back();
        sp.pop_back();
        ans[ind] = mex;
        mex++;
      }
      continue;
    } else if (mex == inp[i]) {
      sp.push_back(i);
    }
  }
  while (sp.size() != 0) {
    int ind = sp.back();
    sp.pop_back();
    ans[ind] = mex + 1;
  }
  for (int i = 0; i < n; i++) {
    cout << ans[i] << " ";
  }
  cout << "\n";
}
