#include <iostream>
#include <vector>


using namespace std;

int main()
{
    int N;
    cin >> N;
    vector<int> a(N+1), b(N+1, -1), aux;
    for(int i = 1; i <= N; i++){
        cin >> a[i];
    }
    if(a[1] > 1){
        cout << "NEM";
        return 0;
    }
    if(a[1] == 1){
        b[1] = 0;
    }

    for(int i = 2; i <= N; i++){
        if(a[i] > i || a[i] < a[i-1]){
            cout << "NEM";
            return 0;
        }
        if(a[i] > a[i-1]){
            b[i] = a[i-1];
            for(int j = a[i-1]+1; j <= a[i]-1; j++){
                aux.push_back(j);
            }
        }
    }

    cout << "IGEN\n";
    int aux_index = 0;
    for(int i = 1; i <= N; i++){
        if(b[i] != -1){
            cout << b[i] << " ";
        }
        else{
            if(aux_index < aux.size()){
                cout << aux[aux_index] << " ";
                aux_index++;
            }
            else{
                cout << 2*N << " ";
            }
        }
    }
    return 0;
}