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