#include <bits/stdc++.h>
using namespace std;
void fastio() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
}
int main() {
  fastio();
  long long int T = 1;
  for (long long int test = 1; test <= T; test++) {
    long long int x, n, m, k, ans, cnt, sum;
    ans = 0;
    cnt = 0;
    sum = 0;
    cin >> n;
    k = 0;
    x = 0;
    long long int a[n + 1], b[n + 1];
    map<long long int, long long int> mp;
    for (long long int i = 0; i < n; i++) {
      cin >> a[i];
      b[i] = -1;
    }
    a[n] = 10000000;
    b[n] = -1;
    n++;
    x = 0;
    for (long long int i = 1; i < n; i++) {
      if (a[i] != a[i - 1]) {
        b[i] = a[i - 1];
        mp[a[i - 1]]++;
      }
    }
    x = 0;
    if (a[0] == 1) {
      b[0] = 0;
      mp[0]++;
    }
    for (long long int i = 0; i < n; i++) {
      if (b[i] == -1) {
        while (mp[x] != 0) x++;
        b[i] = x;
        mp[x]++;
      }
      if (i != n - 1) cout << b[i] << " ";
    }
    cout << endl;
  }
}
