#include<iostream>
#include<string.h>
using namespace std;
const int MAX_N=2000001;
int n;
char str[MAX_N+1];
int main(){
cin >> n >> str;
int i,tmp=0;
for(i=0; i<n/2 && str[i]==str[n/2+i]; i++);
for(; i<n/2 && str[i]==str[n/2+1+i]; i++);
if(i==n/2) tmp+=1;
for(i=0; i<n/2 && str[i]==str[n/2+1+i]; i++);
for(; i<n/2 && str[i+1]==str[n/2+1+i]; i++);
if(i==n/2) tmp+=2;
str[n/2]=0;
if(tmp==0 || n%2==0) cout<<"NOT POSSIBLE";
else if(tmp==3 && strcmp(str,str+n/2+1)) cout << "NOT UNIQUE";
else if(tmp==1) cout<<str;
else cout<<str+n/2+1;
return 0;
}
