#include <bits/stdc++.h>
using namespace std;
double tick() {
  static clock_t oldt, newt = clock();
  double diff = 1.0 * (newt - oldt) / CLOCKS_PER_SEC;
  oldt = newt;
  return diff;
}
long long int fastExpo(long long int a, long long int n) {
  long long int res = 1;
  while (n) {
    if (n % 2) res = res * a;
    n /= 2;
    a = a * a;
  }
  return res;
}
int dx[] = {-1, 0, 1, 0};
int dy[] = {0, 1, 0, -1};
int a[100010], b[100010], vis[100010];
int main() {
  int n;
  cin >> n;
  for (int i = 0; i < n; i++) cin >> a[i];
  memset(b, -1, sizeof(b));
  memset(vis, 0, sizeof(vis));
  for (int i = 0; i < n; i++) {
    if (a[i] != a[i + 1]) {
      b[i + 1] = a[i];
      vis[a[i]] = 1;
    }
  }
  vis[a[0]] = 1;
  int j = 0;
  for (int i = 0; i < n; i++) {
    if (b[i] == -1) {
      while (vis[j]) j++;
      b[i] = j;
      vis[j] = 1;
    }
  }
  for (int i = 0; i < n; i++) cout << b[i] << " ";
  cout << endl;
  cerr << "execution time is: " << tick() << "\n";
}
