#include <bits/stdc++.h>
using namespace std;
long long powa(long long x, long long y) {
  long long ans = 1;
  while (y) {
    if (y & 1) {
      ans = (ans % 1000000007 * x % 1000000007) % 1000000007;
    }
    x = (x % 1000000007 * x % 1000000007) % 1000000007;
    y = y >> 1;
  }
  return ans;
}
void solve() {
  int n, nei = 0, ni = 0, mx = INT_MIN;
  cin >> n;
  vector<int> a(n), b(n, -1);
  for (int i = 0; i < n; ++i) {
    cin >> a[i];
    mx = max(mx, a[i]);
  }
  int c = 0;
  for (int i = 0; i < n; ++i) {
    if (nei > a[i]) {
      c = 1;
      break;
    }
    while (nei != a[i]) {
      int it = lower_bound(a.begin(), a.end(), nei) - a.begin();
      if (a[it] == nei) {
        int it1 = upper_bound(a.begin(), a.end(), nei) - a.begin();
        b[it1] = nei;
        nei++;
      } else {
        while (ni < n && b[ni] != -1) {
          ni++;
        }
        b[ni] = nei;
        nei++;
      }
    }
  }
  if (c == 1) {
    cout << -1;
  } else
    for (int i = 0; i < n; ++i) {
      if (b[i] == -1) {
        b[i] = mx + 1;
        mx++;
      }
      cout << b[i] << " ";
    }
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  int t = 1;
  while (t--) {
    solve();
  }
  return 0;
}
