#include<bits/stdc++.h>
using namespace std;
string u,x,y,v;
long long vis[27],ans,w,j;
int main(){
    long long n;
    cin>>n;
    cin>>u;
    for(long long i=0;i<n;i++){
        int y=u[i]-'A'+1;
        vis[y]++;
    }
    if(n%2==0){
        cout<<"NOT POSSIBLE";
        return 0;
    }
    for(int i=1;i<=26;i++)
        if(vis[i]==1)ans++;
    if(ans>1){
        cout<<"NOT POSSIBLE";
        return 0;
    }
    for(int i=1;i<27;i++){
        if(vis[i]>2&&vis[i]%2==1){
            cout<<"NOT UNIQUE";
            return 0;
        }
    }
    for(long long i=0;i<n;i++){
        if(vis[u[i]-'A'+1]%2==0){
            //x[j++]=u[i];
            v.push_back(u[i]);
        }
    }
    for(long long i=0;i<v.size();i++){
        if(i<v.size()/2)x.push_back(v[i]);
        else y.push_back(v[i]);
    }
    if(x==y)cout<<x;
    else cout<<"NOT POSSIBLE";
    return 0;
    
}
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