#include <bits/stdc++.h>
using namespace std;
long long dx[4] = {0, 0, 1, -1};
long long dy[4] = {1, -1, 0, 0};
void fast() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout << setprecision(12) << fixed;
}
void solve() {
  long long n;
  cin >> n;
  long long a[n + 1];
  a[0] = 0;
  for (long long i = 0; i < n; i++) cin >> a[i + 1];
  long long b[n + 1], vis[n + 5];
  memset(b, (long long)-1, sizeof(b));
  memset(vis, (long long)0, sizeof(vis));
  for (long long i = 0; i < n; i++) {
    if (a[i + 1] != a[i]) {
      b[i] = a[i];
      vis[b[i]]++;
    }
  }
  vis[a[n]]++;
  long long j = 0;
  for (long long i = 0; i < n; i++) {
    if (b[i] == -1) {
      while (vis[j]) j++;
      b[i] = j;
      vis[j] = 1;
    }
  }
  for (long long i = 0; i < n; i++) cout << b[i] << " ";
  cout << endl;
  ;
}
int main() {
  fast();
  long long t = 1;
  while (t--) {
    solve();
  }
}
