#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
#include <bits/stdc++.h>
#define fi first
#define se second
#define pb push_back
#define sz(a) (int)a.size()
#define all(a) a.begin(), a.end()
#define rall(a) a.rbegin(), a.rend()
#define ff(i,a,b) for(int i=a;i<=b;i++)
#define fb(i,b,a) for(int i=b;i>=a;i--)
using namespace std;
using namespace __gnu_pbds;
typedef long long ll;
typedef pair<int,int> pii;
const int maxn = 2000005;
const int inf = 1e9 + 5;
template<typename T>
using ordered_set = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
// os.order_of_key(k) the number of elements in the os less than k
// *os.find_by_order(k) print the k-th smallest number in os(0-based)
int n;
string s;
// Hashing - O(n)
const int mod = 1e9 + 7;
ll add(ll a, ll b){
return (a + b) % mod;
}
ll sub(ll a, ll b){
return (a - b + mod) % mod;
}
ll mul(ll a, ll b){
return (a * b) % mod;
}
ll power(ll a, ll b){
if(!b)return 1;
ll pola = power(a, b / 2);
pola = mul(pola, pola);
if(b % 2)pola = mul(pola, a);
return pola;
}
ll inv(ll a){
return power(a, mod - 2);
}
const int p = 31;
ll pw[maxn];
ll invz[maxn];
void init_hsh(){
pw[0] = 1;
ff(i,1,maxn - 1)pw[i] = mul(pw[i - 1], p);
invz[maxn - 1] = inv(pw[maxn - 1]);
fb(i,maxn - 2, 0)invz[i] = mul(invz[i + 1], p);
}
ll hsh[maxn];
ll get(int l, int r){
if(l > r)return 0;
return mul(sub(hsh[r], hsh[l - 1]), invz[l - 1]);
}
int main()
{
ios::sync_with_stdio(false);
cout.tie(nullptr);
cin.tie(nullptr);
cin >> n >> s;
if(n % 2 == 0)return cout << "NOT POSSIBLE", 0;
init_hsh();
ff(i,0,n - 1)hsh[i + 1] = add(hsh[i], mul(pw[i], s[i] - 'A' + 1));
int len = (n - 1) / 2;
int rez = 0;
int L = 0, R = -1;
ff(i,1,len){
// brisem i-ti
ll a1 = hsh[i - 1];
ll a2 = mul(pw[i - 1], get(i + 1, len + 1));
ll a = a1 + a2;
ll b = get(len + 2, n);
if(a == b){
rez += 1;
L = len + 2;
R = n;
}
}
ff(i,len + 1,n){
// brisem i-ti
ll a1 = get(len + 1, i - 1);
ll a2 = mul(pw[i - len - 1], get(i + 1, n));
ll a = a1 + a2;
ll b = hsh[len];
if(a == b){
rez += 1;
L = 1;
R = len;
}
}
if(rez == 0){
cout << "NOT POSSIBLE" << endl;
return 0;
}
if(rez > 1){
cout << "NOT UNIQUE" << endl;
return 0;
}
ff(i,L - 1,R - 1)cout << s[i];
cout << endl;
return 0;
}
/**
// probati bojenje sahovski ili slicno
**/
