#include <bits/stdc++.h>
using namespace std;
long long int max(long long int a, long long int b) {
  if (a > b) return a;
  return b;
}
long long int min(long long int a, long long int b) {
  if (a < b) return a;
  return b;
}
long long int power(long long int x, long long int y,
                    long long int m = 1000000007) {
  if (y < 0) {
    x = power(x, m - 2);
    y = -y;
  }
  long long int ans = 1;
  x = x % m;
  while (y) {
    if (y & 1) ans = (ans * x) % m;
    y = y / 2;
    x = (x * x) % m;
  }
  return ans;
}
long long int factorial(long long int n, long long int m = 1000000007) {
  long long int ans = 1;
  long long int i = 1;
  while (i <= n) {
    ans = (ans * i) % m;
    i++;
  }
  return ans;
}
long long int vis[1000000] = {0};
int main() {
  ios_base::sync_with_stdio(false);
  long long int n;
  cin >> n;
  long long int arr[n];
  long long int flag = 1;
  for (long long int i = 0; i < n; i++) {
    cin >> arr[i];
    if (i + 1 < arr[i]) flag = 0;
    vis[arr[i]] = 1;
  }
  if (flag == 0) {
    cout << "-1\n";
    return 0;
  }
  vector<long long int> ans;
  long long int p = 0;
  for (long long int i = 0; i < n; i++) {
    if (i != 0 && arr[i] != arr[i - 1]) {
      ans.push_back(arr[i - 1]);
    } else {
      while (vis[p] == 1) p++;
      ans.push_back(p);
      p++;
    }
  }
  long long int len = ans.size();
  for (long long int i = 0; i < len; i++) {
    cout << ans[i] << " ";
  }
  cout << "\n";
  return 0;
}
