#include<bits/stdc++.h>
using namespace std;
int main(){
int n; cin>>n;
int k=n/2;
string sa,sb,s; cin>>s;
if(n%2==0){
cout<<"NOT POSSIBLE";
return 0;
}
for(int i=0;i<k;i++) sa.push_back(s[i]);
for(int i=n-k;i<n;i++) sb.push_back(s[i]);
if(sa==sb){
cout<<sa;
return 0;
}
int id=0,c=0;
string ans="67";
for(int i=k;i<n;i++){
if(s[i]==sa[id]) id++;
if(id==k) break;
}
if(id==k) c++,ans=sa;
id=0;
for(int i=0;i<n-k;i++){
if(s[i]==sb[id]) id++;
if(id==k) break;
}
if(id==k) c++,ans=sb;
if(ans=="67") cout<<"NOT POSSIBLE";
else if(c==1) cout<<ans;
else cout<<"NOT UNIQUE";
}
