#include <bits/stdc++.h>
using namespace std;
long long a[1000005];
long long b[1000005];
long long tp[5000005];
long long pp[1000005];
long long cc[1000005];
vector<long long> V;
priority_queue<long long, vector<long long>, greater<long long>> Q;
int main() {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);
  long long n;
  cin >> n;
  int bj = 0;
  long long minn = 0;
  long long mi = 0;
  for (int i = 1; i <= n; i++) {
    cin >> a[i];
    tp[a[i]]++;
  }
  for (long long i = 0; i <= 3 * n; i++) {
    if (tp[i] == 0) {
      Q.push(i);
    }
  }
  long long cnt = 0;
  for (int i = 1; i <= n; i++) {
    b[i] = Q.top();
    Q.pop();
    tp[a[i]]--;
    if (tp[a[i]] == 0) {
      Q.push(a[i]);
    }
  }
  long long M = 0;
  for (int i = 1; i <= n; i++) {
    cc[b[i]]++;
    while (cc[M] > 0) {
      M++;
    }
    if (M < a[i]) {
      bj = 1;
      break;
    }
  }
  if (bj == 1) {
    cout << "-1\n";
  } else {
    for (int i = 1; i <= n; i++) {
      cout << b[i] << " ";
    }
    cout << "\n";
  }
  return 0;
}
