#include <bits/stdc++.h>
#define subs(a, b) substr(a, b-a+1)
using namespace std;
int main() {
int n;
cin >> n;
if(n%2 == 0){
cout << "NOT POSSIBLE\n";
return 0;
}
string str;
cin >> str;
string resp = "NOT POSSIBLE";
for(int i=0;i<n;i++){
string x;
for(int j=0;j<n;j++)
if(j != i) x += str[j];
int k = x.size();
if(x.subs(0, k/2 - 1) == x.subs(k/2, k-1)){
if(resp != "NOT POSSIBLE" && resp != x.subs(0, k/2 - 1)){
cout << "NOT UNIQUE\n";
return 0;
}
resp = x.subs(0, k/2 - 1);
}
}
cout << resp << "\n";
}
