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

int main() {
    ios::sync_with_stdio(0); cin.tie(0);
    int n; cin >> n;
    vector<int> a(n + 1, 0), b(n + 1), f;
    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) {
            cout << "NEM\n"; return 0;
        }
    }
    for (int i = 1; i <= n; i++) {
        if (a[i] > a[i - 1]) {
            b[i] = a[i - 1];
            for (int j = a[i - 1] + 1; j < a[i]; j++) {
                if (f.empty()) { cout << "NEM\n"; return 0; }
                b[f.back()] = j; f.pop_back();
            }
        } else f.push_back(i);
    }
    int v = n + 1;
    while (!f.empty()) { b[f.back()] = v++; f.pop_back(); }
    cout << "IGEN\n";
    for (int i = 1; i <= n; i++) cout << b[i] << (i == n ? "" : " ");
    cout << endl;
    return 0;
}