#include<bits/stdc++.h>
using namespace std;
bool vis[1000005];
int arr[1000005], n, ind;
int main()
{
    freopen("prefmex.in", "r", stdin);
    freopen("prefmex.out", "w", stdout);
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    cout.tie(NULL);
    cin>>n;
    for(int i = 0; i < n; i ++)
    {
        cin>>arr[i];
    }
    ind = 0;
    for(int i = 0; i < n; i ++)
    {
        vis[arr[i]] = 1;
        while(vis[ind] == 1) ind ++;
        cout<<ind<<' ';
    }
    cout<<endl;
    return 0;
}