#include <bits/stdc++.h>
using namespace std;
const long long oo = 1e18;
const double PI = acos(-1);
const long long M = 1e9 + 7;
const int N = 10000010;
long long oor(long long a, long long b) { return (a | b); }
int a[100010], b[100010], use[100010];
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  int n;
  cin >> n;
  for (int i = 0; i <= n; ++i) b[i] = -1;
  for (int i = 0; i < n; ++i) {
    cin >> a[i];
    if (i && a[i] != a[i - 1])
      b[i] = a[i - 1], use[a[i - 1]] = 1;
    else if (i == 0 && a[i])
      b[i] = 0, use[0] = 1;
  }
  use[a[n - 1]] = 1;
  int co = 0;
  for (int i = 0; i < n; ++i) {
    while (use[co]) co++;
    if (b[i] == -1) b[i] = co, use[co] = 1;
    cout << b[i] << ' ';
  }
}
