#include<bits/stdc++.h>
using namespace std;
void speed()
{
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    cout.tie(NULL);
}
int n,a[1000005],me,used[1000005];
void read()
{
    cin>>n;
    for(int i=1;i<=n;i++)
    {
        cin>>a[i];
        used[a[i]]=1;
        while(used[me])me++;
        cout<<me<<" ";
    }
    cout<<endl;
}
int main()
{
    #ifdef ONLINE_JUDGE
    freopen("prefmex.in", "r", stdin);
    freopen("prefmex.out", "w", stdout);
#endif
    speed();
    read();
    return 0;
}