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