#include<bits/stdc++.h>
using namespace std;
/*
#include <ext/pb_ds/assoc_container.hpp>
using namespace __gnu_pbds;
template <class T>
using Tree = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
*/
#ifdef LOCAL
#include "algo/debug.h"
#else
#define debug(...) 42
#endif
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define rep1(i, n) for (int i = 1; i < (n); ++i)
#define rep1n(i, n) for (int i = 1; i <= (n); ++i)
#define repr(i, n) for (int i = (n) - 1; i >= 0; --i)
#define fr first
#define sc second
#define pb push_back
#define mp make_pair
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
#define endl '\n'
using ll = long long;
using ull = unsigned long long;
using ld = long double;
using str = string;
using pi = pair<int, int>;
using pl = pair<ll, ll>;
using vi = vector<int>;
using vl = vector<ll>;
using vpi = vector<pair<int, int>>;
using vvi = vector<vi>;
const int nax = (int)2000002;
const int m = (int)1e9 + 9;
const int p = 9973;
ll bpow(ll a, ll b){
ll res = 1;
while(b){
if (b & 1)res = (res * a)%m;
b /= 2;
a = (a * a)%m;
}
return res;
}
void solve(){
int n;
string s;
cin >> n >> s;
if (n % 2 == 0){
cout << "NOT POSSIBLE" << endl;
return;
}
ll hsh[n+1];
hsh[0] = 0;
for (int i = 0; i < n; i++){
hsh[i + 1] = (hsh[i] * p % m + (s[i] - 'A' + 1))%m;
// cout << hsh[i + 1] << endl;
}
ll ps[n + 1];
ps[0] = 1;
for (int i = 1; i <= n; i++){
ps[i] = p * ps[i - 1];
ps[i] %= m;
}
function<ll(int,int)>get = [&](int a, int b){
return (hsh[b + 1] - hsh[a] * ps[b - a + 1]%m + m)%m;
};
unordered_set<ll>se;
string res = "";
for (int i = 0; i < n; i++){
ll hash1 = 0, hash2 = 0;
if (i == 0){
hash1 = get(1,n / 2);
hash2 = get(n/2+1,n - 1);
}
else if (i == n / 2){
hash1 = get(0,i-1);
hash2 = get(i+1,n - 1);
}else if (i < n / 2){
hash1 = (get(0, i - 1) * ps[n / 2 - i] + get(i + 1, n / 2))%m;
hash2 = get(n - n / 2, n - 1);
}else{
hash1 = get(0, n / 2 - 1);
hash2 = get(n / 2, i - 1) * ps[n / 2 - (i - 1 - (n / 2) + 1)] % m + get(i + 1, n - 1);
}
hash1 %= m;
hash2 %= m;
debug(hash1,hash2);
if (hash1 == hash2){
if (se.empty()){
for (int j = 0; j < n; j++){
if (i == j)continue;
if ((int)res.size() < n / 2)res += s[j];
else break;
}
}
se.insert(hash1);
if (se.size() > 1){
cout << "NOT UNIQUE" << endl;
return;
}
}
}
if (res.size() == 0)cout << "NOT POSSIBLE" << endl;
else cout << res << endl;
}
int main(){
ios::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
int tt = 1;
//cin >> tt;
while(tt--){
solve();
}
return 0;
}
