#include<iostream>
#include<vector>
using namespace std;

int n;
int pref[1000005];
int ans[1000005];
bool used[1000005];

void solve(){
    int mex = 0;
    int iter = 0;
    for(int i = 0; i < n; i++){
        used[pref[i]] = true;
        while(used[iter] == true) iter++;
        mex = iter;
        ans[i] = mex;
    }
    for(int i = 0; i < n; i++){
        std::cout<<ans[i]<<" ";
    }
    cout<<"\n";
}

int main(){
    freopen("prefmex.in","r",stdin);
    freopen("prefmex.out","w",stdout);
    ios_base::sync_with_stdio(false);
    cin>>n;
    int p;
    for(int i = 0; i < n; i++){
        cin>>pref[i];
    }
    solve();
}