#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define rep(i, n) for(ll i = 0, i##_len = (n); i < i##_len; i++)
#define reps(i, s, n) for(ll i = (s), i##_len = (n); i < i##_len; i++)
#define rrep(i, n) for(ll i = (n) - 1; i >= 0; i--)
#define rreps(i, e, n) for(ll i = (n) - 1; i >= (e); i--)
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
#define sz(x) ((ll)(x).size())
#define len(x) ((ll)(x).length())
#define endl "\n"

int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    // ifstream in("input.txt");
    // cin.rdbuf(in.rdbuf());
    ll n;
    cin >> n;
    vector<ll> a(n);
    rep(i, n) cin >> a[i];
    set<ll> dislike;
    rep(i, n) dislike.insert(a[i]);
    set<ll> cand;
    rep(i, n) cand.insert(i);
    vector<ll> ans(n);
    rep(i, n) {
        ll sel = -1;
        for(auto x : dislike) {
            if (x == a[i]) continue;
            sel = x;
        }
        if (sel != -1) {
            ans[i] = sel;
            dislike.erase(sel);
            cand.erase(sel);
            continue;
        }
        for(auto x : cand) {
            if (x == a[i]) continue;
            sel = x;
        }
        if (sel == -1) {
            printf("-1\n");
            return 0;
        }
        ans[i] = sel;
        cand.erase(sel);
        dislike.erase(sel);
    }
    rep(i, n) printf("%lld\n", ans[i]);
    return 0;
}