#include <algorithm>
#include <iostream>
#include <numeric>
#include <cassert>
#include <vector>
#include <bitset>
#include <random>
#include <queue>
#include <set>

typedef long long llong;
const int MAXN = 1000000 + 10;
const int INTINF = 2e9;
const llong INF = 1e18;
const int MAXLOG = 17;

int n;
int a[MAXN];
bool count[MAXN];

void solve()
{
    int ptr = 0;
    for (int i = 1 ; i <= n ; ++i)
    {
        count[a[i]] = true;
        while (count[ptr]) ptr++;
        std::cout << ptr << ' ';
    }

    std::cout << '\n';
}

void input()
{
    std::cin >> n;
    for (int i = 1 ; i <= n ; ++i)
    {
        std::cin >> a[i];
    }
}

void fastIOI()
{   
    freopen("prefmex.in", "r", stdin);
    freopen("prefmex.out", "w", stdout);
    std::ios_base :: sync_with_stdio(0);
    std::cout.tie(nullptr);
    std::cin.tie(nullptr);
}

int main()
{
    fastIOI();
    input();
    solve();

    return 0;
}