#include <bits/stdc++.h>

using ll = long long;
using namespace std;

const int MAXN = 500001;
int v[MAXN], b[MAXN], ans[MAXN];

int main()
{
    ios::sync_with_stdio(0); cin.tie(0);
    int n; cin >> n;
    fill(ans, ans+MAXN, n+1);
    for (int i = 1; i <= n; i++) {
        cin >> v[i];
    }
    for (int i = 2; i <= n; i++) {
        if (v[i-1] > v[i]) {
            cout << "NEM\n";
            return 0;
        }
    }
    vector<int> idx;
    int x = 0;
    for (int i = 1; i <= n; i++) {
        idx.push_back(i);
        if (v[i] != v[i-1]) {
            for (; x < v[i]; x++) {
                if (idx.empty()) {
                    cout << "NEM\n";
                    return 0;
                }
                ans[idx.back()] = x;
                idx.pop_back();
            }
        }
    }

    cout << "IGEN\n";
    for (int i = 1; i <= n; i++) {
        cout << ans[i] << " ";
    }
    cout << "\n";
    return 0;
}