#include<bits/stdc++.h>
using namespace std;
int mas[1000010];
bool br[1000010];
int main(){
    freopen("prefmex.in","r",stdin);
    freopen("prefmex.out","w",stdout);
    int N;
    cin>>N;
    for(int i=0;i<N;i++){
        cin>>mas[i];
    }
    int seg=0;
    for(int i=0;i<N;i++){
        br[mas[i]]=1;
        while(br[seg]){
            seg++;
        }
        cout<<seg;
        if(i!=N-1){
            cout<<" ";
        }
    }
    return 0;
}