#include <bits/stdc++.h>
using namespace std;
//#define endl '\n'
#pragma GCC optimize("O3" , "Ofast" , "unroll-loops" , "fast-math")
#pragma GCC target(avx2)
int n;
int t[1000005];
int ans[1000005];
int used[1000005];
void speed()
{
    ios_base::sync_with_stdio(NULL);
    cin.tie(nullptr);
    cout.tie(nullptr);
}
void read()
{
    cin >> n;
    for(int i = 0 ; i < n ; i++)
    {
        cin >> t[i];
    }
}
void print()
{
    for(int i = 0 ; i < n ; i++)
        cout << used[i] << ' ';
    cout <<endl;
}
void solve()
{
    int min_ans = 0;
    for(int i = 0 ; i < n ; i++)
    {
        used[t[i]] = 1;
        int k = min_ans;
        if(t[i] == min_ans)
        {
            while(used[k] != 0)
            {
                k++;
            }
            min_ans = k;
        }
            cout << k << " ";
    }
}
int main()
{
    #ifdef ONLINE_JUDGE
    freopen("prefmex.in", "r", stdin);
    freopen("prefmex.out", "w", stdout);
    #endif
    speed();
    read();
    solve();

    return 0;
}