#include <bits/stdc++.h>

using namespace std;

int main()
{
    int n;
    cin >> n;
    vector<int> szamok;
    vector<int> valt;
    bool vege=false;
    for (int i=0; i<n; i++) {
        int x;
        cin >> x;
        szamok.push_back(x);
        if (szamok[i]>i+1) {
            vege=true;
        }
    }
    if (vege==true) {
        cout << "NEM\n";
        return 0;
    }
    for (int i=1; i<n; i++) {
        if (szamok[i]<szamok[i-1]) {
            cout << "NEM\n";
            return 0;
        }
        if (szamok[i]>szamok[i-1]) {
            valt.push_back(szamok[i]);
        }
    }

    int epp=0;
    vector<int> veg;
    int y=0;
    if (szamok[0]==0) {
        epp++;
        while (epp==valt[y]) {
            epp++;
            y++;
        }
        veg.push_back(epp);
    }
    else  {
        veg.push_back(epp);
        epp++;
    }
    for (int i=1; i<n; i++) {
        if (szamok[i]>szamok[i-1]) {
             veg.push_back(szamok[i-1]);
        }
        else {
            epp++;
            while (epp==valt[y]) {
                epp++;
                y++;
            }
            veg.push_back(epp);
        }
    }
    cout << "IGEN\n";
    for (int sz : veg) {
        cout << sz << " ";
    }
    return 0;
}