#include<iostream>
#include<stdio.h>
using namespace std;
const int MAXN = 1e6+7;
bool cnt[MAXN];
int main () {
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
    cout.tie(nullptr);
    freopen("prefmex.in","r",stdin);
    freopen("prefmex.out","w",stdout);
    long long n;
    cin>>n;
    long long a,j=0;
    for(int i=0;i<n;i++){
        cin>>a;
        cnt[a]=true;
        while(cnt[j]){
            j++;
        }
        if(i>0) cout<<" ";
        cout<<j;
    }
    cout<<"\n";
    return 0;
}