// author: Yusif_Nazarli
#pragma GCC optimize("O3")
#include "bits/stdc++.h"
#include "ext/pb_ds/assoc_container.hpp"
#define Mor_Te ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
#define gcd __gcd
#define intt long long
#define pb push_back
#define emb emplace_back
#define all(v) v.begin(), v.end()
#define rall(v) v.rbegin(), v.rend()
#define F first
#define S second
#define pii pair<intt ,intt>
#define reset_map(m) for(auto &[k , v] : m) v = 0;
using namespace std;
using namespace __gnu_pbds;
template <typename T>
using indexed_set = tree<T , null_type , less<T> , rb_tree_tag , tree_order_statistics_node_update>;
const intt sz = 1e5 + 5;
const intt INF = 1e18;
void Yusiff(){
intt n; cin >> n;
string s; cin >> s;
set<string> st;
for(intt i = 0; i < n; i++){
string temp = s;
temp.erase(i , 1);
if(temp.substr(0 , temp.size() / 2) == temp.substr(temp.size() / 2 , temp.size() / 2)){
st.insert(temp.substr(0 , temp.size()));
}
}
if(st.size() == 0){
cout << "NOT POSSIBLE" << '\n';
} else if(st.size() == 1){
cout << *st.begin() << '\n';
} else{
cout << "NOT UNIQUE" << '\n';
}
}
signed main(){
int t = 1;
//cin >> t;
for(intt i = 1; i <= t; i++){
// cout << "Case " << i << ": ";
Yusiff();
}
}
