#include <iostream>
#include <vector>
#include <queue>
using namespace std;

void output_vector(vector<int> vec, bool isEnterSeperated) {
    for (size_t i = 0; i < vec.size(); i++)
    {
        cout << vec[i];
        if (isEnterSeperated) {
            cout << "\n";
        }
        else {
            cout << " ";
        }
    }
}

int main()
{
    int n;
    cin >> n;
    vector<int> res(n,-1);
    vector<int> inp(n);
    vector<bool> app(2*n);
    int old = -1;
    for (size_t i = 0; i < n; i++)
    {
        cin >> inp[i];
        if (inp[i] > i + 1) {
            cout << "NEM";
            return 0;
        }
        app[inp[i]] = true;
        if (inp[i] != old) {
            res[i] = old;
        }
        old = inp[i];
    }
    int l = 0;
    for (size_t i = 0; i < res.size(); i++)
    {
        if (res[i] == -1) {
            for (size_t j = l; j < app.size(); j++)
            {
                if (!app[j]) {
                    l = j;
                    break;
                }
            }
            res[i] = l;
            l++;
        }
    }
    cout << "IGEN\n";
    output_vector(res, false);
}