#include <bits/stdc++.h>
using namespace std;
using ll = long long;
int main(){
ios::sync_with_stdio(false);
cin.tie(nullptr);
ll n;
string s;
cin >> n >> s;
bool check = false;
string ans = "";
if (n % 2 == 0){
cout << "NOT POSSIBLE\n";
return 0;
}
for (int i = 0; i < n; i++){
string x = "";
for (int j = 0; j < n; j++){
if (j != i){
x += s[j];
}
}
if (x.substr(0, n / 2) == x.substr(n / 2)){
if (check) {
cout << "NOT UNIQUE\n";
return 0;
}
if (!check){
check = true;
ans = x.substr(0, n / 2);
}
}
}
cout << (check ? ans : "NOT POSSIBLE\n");
return 0;
}
