#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, a[100005];
  while (scanf("%d", &n) == 1) {
    for (int i = 0; i < n; i++) {
      scanf("%d", &a[i]);
    }
    int b[n + 2];
    memset(b, -1, sizeof(b));
    set<int> SET;
    for (int i = 0; i < a[n - 1]; i++) SET.insert(i);
    for (int i = 0; i < n; i++) {
      if (i == 0) {
        if (a[i] > 0) b[i] = 0, SET.erase(0);
      } else if (a[i - 1] < a[i])
        b[i] = a[i - 1], SET.erase(a[i - 1]);
    }
    for (int i = 0; i < n; i++) {
      if (b[i] == -1) {
        if (SET.size() == 0)
          b[i] = n + 1;
        else {
          b[i] = *SET.begin();
          SET.erase(SET.begin());
        }
      }
    }
    for (int i = 0; i < n; i++) {
      printf("%d ", b[i]);
    }
    printf("\n");
    SET.clear();
  }
  cerr << "\nTime : " << 1.0 * clock() / CLOCKS_PER_SEC << " sec.\n";
  return 0;
}
