#include<bits/stdc++.h>
using namespace std;
string u;
int vis[27],ans,w;
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=0;i<n;i++){
if(w>=n/2)return 0;
if(vis[u[i]-'A'+1]%2==0)cout<<char('A'+i-1);
}
return 0;
}
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