#include<bits/stdc++.h>
using namespace std;
int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(0);
    int n;
    cin>>n;
    vector<int> a(n);
    for(int i=0;i<n;++i) {
        cin>>a[i];
    }
    bool ok=true;
    for(int i=0;i<n;++i) {
        ok&=i==0||a[i-1]<=a[i];
        ok&=a[i]<=i+1;
    }
    
    cerr<<ok<<"\n";
    if(!ok) {
        cout<<"NEM\n";
        return 0;
    }

    a.insert(a.begin(), 0);
    int mx=*max_element(a.begin(), a.end());

    vector<int> had(2*n);
    vector<int> b(n+1,-1);
    for(int i=1;i<=n;++i) {
        if(a[i-1]!=a[i]) {
            b[i]=a[i-1];
            had[b[i]]=1;
        }
    }
    
    int curr=0;
    for(int i=1;i<=n;++i) {
        if(b[i]==-1) {
            while(had[curr] || curr==mx) curr++;
            b[i]=curr;
            had[curr]=1;
        }
    }

    for(int i=1;i<=n;) {
        int j=i;
        while(j<=n && a[i]==a[j]) j++;
        if(i+1<j) {
            reverse(b.begin()+i+1, b.begin()+j);
        }
        i=j;
    }

    cout<<"IGEN\n";
    for(int i=1;i<=n;++i) cout<<b[i]<<" ";
    cout<<"\n";
    return 0;
}