#include <bits/stdc++.h>
#pragma GCC optimize("O3", "unroll-loops")
#define pb emplace_back
#define ins insert
#define ssize(x) (int)x.size()
#define all(x) x.begin(), x.end()
#define rall(x) x.rbegin(), x.rend()
#define pii pair <int, int>
#define pll pair <ll, ll>
#define pld pair <ld, ld>
#define st first
#define nd second
using namespace std;
using ll = int_fast64_t;
// using lll = __int128_t;
using ld = long double;
const int oo = 1e9 + 7;
const int mod = 1e9 + 9;
struct Hash{
int n, p = 2137;
vector <ll> h, pw;
Hash(string &s){
n = s.length();
pw.resize(n + 1);
pw[0] = 1;
for(int i = 1; i <= n; i ++) pw[i] = (pw[i - 1] * p) % mod;
h.resize(n + 1);
for(int i = 1; i <= n; i ++) h[i] = (h[i - 1] + (pw[i] * (ll)(s[i - 1] - 'A' + 1)) % mod) % mod;
}
ll fp(ll a, ll b){
ll r = 1;
while(b > 0){
if(b & 1) r = (r * a) % mod;
a = (a * a) % mod;
b >>= 1;
}
return r;
}
ll get(int a, int b){
ll r = (h[b] - h[a - 1] + mod) % mod;
r = (r * fp(pw[a], mod - 2)) % mod;
return r;
}
ll gett(int a, int b, int c, int d){
ll h1 = get(a, b), h2 = get(c, d);
ll r = (h1 + (h2 * pw[b - a + 1]) % mod) % mod;
return r;
}
};
void solve(){
int n; cin >> n;
string s; cin >> s;
if(!(n & 1)){
cout << "NOT POSSIBLE\n";
return;
}
Hash H(s);
vector <int> ev;
int k = (n + 1) / 2, l = n / 2;
if(H.get(2, 2 + l - 1) == H.get(n - l + 1, n)) ev.pb(1);
if(H.get(1, 1 + l - 1) == H.get(n - l, n - 1)) ev.pb(n);
for(int i = 2; i < n; i ++){
if(i == k){
if(H.get(1, k - 1) == H.get(k + 1, n)) ev.pb(i);
}
if(i < k){
if(H.get(n - l + 1, n) == H.gett(1, i - 1, i + 1, k)) ev.pb(i);
}
if(i > k){
if(H.get(1, l) == H.gett(k, i - 1, i + 1, n)) ev.pb(i);
}
}
if(!ssize(ev)){
cout << "NOT POSSIBLE\n";
return;
}
int idx = ev[0];
if(ssize(ev) > 1){
ll hsh = 0;
bool ok = 1;
if(idx >= k) hsh = H.get(1, l);
else hsh = H.get(k + 1, n);
for(int i = 1; i < ssize(ev); i ++){
if(ev[i] >= k && hsh != H.get(1, l)) ok = 0;
if(ev[i] < k && hsh != H.get(k + 1, n)) ok = 0;
}
if(!ok){
cout << "NOT UNIQUE\n";
return;
}
}
if(idx >= k){
for(int i = 1; i <= l; i ++) cout << s[i - 1];
}
else{
for(int i = n - l + 1; i <= n; i ++) cout << s[i - 1];
}
cout << '\n';
}
signed main(){
ios_base::sync_with_stdio(0); cin.tie(0);
int t; t = 1;
// cin >> t;
while(t --) solve();
return 0;
}
