#include <bits/stdc++.h>
using namespace std;

#define ll long long
#define pb push_back
#define mp make_pair
#define pii pair<int, int>
#define fi first
#define se second

void solve() {
    int n;
    cin >> n;
    vector<int> a(1 + n, 0);
    for(int i = 1; i <= n; i++) {
        cin >> a[i];
    }
    for(int i = 1; i <= n; i++) {
        if(a[i] < a[i - 1] || a[i] >= i + 1) {
            cout << "NEM\n";
            return;
        }
    }
    vector<bool> used(1 + 2 * n, false);
    for(int i = 0; i <= n; i++) {
        used[a[i]] = true;
    }
    vector<int> b(1 + n, 0);
    int nxt_unused = 0;
    for(int i = 1; i <= n; i++) {
        if(a[i] != a[i - 1]) {
            b[i] = a[i - 1];
        } else {
            while(used[nxt_unused]) {
                nxt_unused++;
            }
            b[i] = nxt_unused;
            nxt_unused++;
        }
    }
    for(int i = 1; i <= n; i++) {
        cout << b[i] << " ";
    }
    cout << "\n";
}

signed main() {
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
    solve();
}