#include <bits/stdc++.h>
using namespace std;
const long double PI = acos(-1);
long long power(long long b, long long e, long long m) {
  if (e == 0) return 1;
  if (e & 1) return b * power(b * b % m, e / 2, m) % m;
  return power(b * b % m, e / 2, m);
}
long long power(long long b, long long e) {
  if (e == 0) return 1;
  if (e & 1) return b * power(b * b, e / 2);
  return power(b * b, e / 2);
}
template <typename T, typename U>
static inline void amin(T &x, U y) {
  if (y < x) x = y;
}
template <typename T, typename U>
static inline void amax(T &x, U y) {
  if (x < y) x = y;
}
template <typename T, typename U>
ostream &operator<<(ostream &os, const pair<T, U> &p) {
  return os << '(' << p.first << "," << p.second << ')';
}
int _runtimeTerror_() {
  long long n;
  cin >> n;
  long long a[n];
  for (int i = 0; i < n; ++i) cin >> a[i];
  long long ans[n];
  memset(ans, -1, sizeof(ans));
  int cur = 0, p = 0;
  bool mark[1000005];
  memset(mark, false, sizeof(mark));
  if (a[0] == 1) {
    ans[0] = 0, ++cur;
    mark[0] = true;
    mark[1] = true;
  } else {
    mark[0] = true;
  }
  for (int i = 1; i < n; ++i) {
    if (a[i] != a[i - 1]) {
      ans[i] = a[i - 1];
      mark[a[i - 1]] = true;
      mark[a[i]] = true;
    } else {
      ++p;
    }
  }
  long long curp = 0;
  for (int i = 0; i < n; ++i) {
    if (ans[i] == -1) {
      while (mark[curp]) ++curp;
      ans[i] = curp;
      ++curp;
    }
  }
  for (int i = 0; i < n; ++i) cout << ans[i] << " ";
  return 0;
}
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  int TESTS = 1;
  while (TESTS--) _runtimeTerror_();
  cerr << "\n" << 1.0 * clock() / CLOCKS_PER_SEC;
  return 0;
}
