#include <iostream>
#include <fstream>

using namespace std;
int n, a[500001];
int b[500001] = { -1 };
int sz = 0, L[500001], l,ma=0;

int main()
{
    //ifstream f("be.in");
    cin >> n;
    for (int i = 1; i <= n; i++) b[i] = -1;
    cin >> a[1];
    if (a[1] == 1) b[1] = 0;
    if (a[1] > ma) ma = a[1];
    for (int i = 2; i <= n; i++) {
        cin >> a[i];
        if (a[i] > ma) ma = a[i];
        if (a[i]<a[i - 1] || a[i]>i) {
            sz = 1;
        }
        else {
            if (a[i] > a[i - 1]) {
                b[i] = a[i - 1];
                for (int j = a[i - 1] + 1; j < a[i]; j++) {
                    l++;
                    L[l] = j;
                }
            }
        }
    }
    if (ma > n) sz = 1;
    if (sz == 1) cout << "NEM";
    else {
        cout << "IGEN" << endl;
        int t = 1;
        for (int i = 1; i <= n; i++) {
            if (b[i] >= 0) cout << b[i] << " ";
            else {
                if (t <= l) {
                    cout << L[t] << " ";
                    t++;
                }
                else cout << n * 2 << " ";
            }
        }
    }
    return 0;
}