#include<bits/stdc++.h>
using namespace std;
int main()
{
    int n,i;
    cin>>n;
    int a[n+1],b[n+1];
    for(i=0;i<n;i++){
        cin>>a[i];
        if(a[i]>i+1){cout<<-1;return 0;}}iota(b,b+n+1,0);
    for(i=n-1;i>=0;i--)if(b[i]>=a[i])swap(b[i+1],b[a[i]]);
    for(i=0;i<n;i++)cout<<b[i]<<" ";
}