#include<bits/stdc++.h>
#define endl '\n'

using namespace std;

const int MAXN=1e6+5;

int n;
bool used[MAXN];
int ptr=0;

int main()
{
ios::sync_with_stdio(false);
cin.tie(NULL);
cout.tie(NULL);

freopen("prefmex.in","r",stdin);
freopen("prefmex.out","w",stdout);

cin>>n;
for(int i=0;i<n;i++)
{
    int a;
    cin>>a;
    used[a]=1;
    while(used[ptr]) ptr++;
    cout<<ptr<<" ";
}
cout<<endl;

return 0;
}