#include <bits/stdc++.h>
using namespace std;
const int MX = 10010896;
const int lmt = 3164;
const int N = 10000001;
const int MOD = 1000000007;
using namespace std;
long long int extgcd(long long int a, long long int b, long long int& x,
                     long long int& y) {
  if (b == 0) {
    x = 1;
    y = 0;
    return a;
  } else {
    int g = extgcd(b, a % b, y, x);
    y -= a / b * x;
    return g;
  }
}
long long int modpow(long long int a, long long int b) {
  long long int res = 1;
  a %= MOD;
  for (; b; b >>= 1) {
    if (b & 1) res = res * a % MOD;
    a = a * a % MOD;
  }
  return res;
}
inline void readi(int& n) {
  n = 0;
  char c = getchar();
  while (c < '0' or c > '9') c = getchar();
  while (c >= '0' and c <= '9') {
    n = (n << 3) + (n << 1) + c - '0';
    c = getchar();
  }
}
inline void readl(long long int& n) {
  n = 0;
  char c = getchar();
  while (c < '0' or c > '9') c = getchar();
  while (c >= '0' and c <= '9') {
    n = (n << 3) + (n << 1) + c - '0';
    c = getchar();
  }
}
inline void print(long long int a) {
  char s[20];
  int i = 0;
  do {
    s[i++] = a % 10 + '0';
    a /= 10;
  } while (a);
  i--;
  while (i >= 0) putchar(s[i--]);
  putchar(' ');
}
inline void printl(long long int a) {
  char s[20];
  int i = 0;
  do {
    s[i++] = a % 10 + '0';
    a /= 10;
  } while (a);
  i--;
  while (i >= 0) putchar(s[i--]);
  putchar('\n');
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  int n;
  cin >> n;
  int* a = new int[n];
  for (int i = (int)0; i < (int)n; ++i) cin >> a[i];
  int flag = 1;
  for (int i = (int)0; i < (int)n; ++i) {
    if (a[i] > (i + 1)) {
      flag = 0;
    }
  }
  if (!flag)
    cout << -1;
  else {
    int* ans = new int[n];
    for (int i = (int)0; i < (int)n; ++i) ans[i] = -1;
    set<int> found;
    for (int i = (int)n - 1; i >= (int)0; --i) {
      if (found.find(a[i]) == found.end()) {
        if (i != n - 1) ans[i + 1] = a[i];
        found.insert(a[i]);
      }
    }
    int hold = 0;
    for (int i = (int)0; i < (int)n; ++i) {
      while (found.find(hold) != found.end()) hold++;
      if (ans[i] != -1) continue;
      ans[i] = hold;
      hold++;
    }
    for (int i = (int)0; i < (int)n; ++i) cout << ans[i] << " ";
  }
}
