#include <bits/stdc++.h>

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

int main()
{
    //ifstream f("be.in");
    cin>>n;
    for (int i=1; i<=n; i++) b[i]=-1;
    cin>>a[1];
    for (int i=2; i<=n; i++) {
        cin>>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 (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;
}