#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;
// Double Hashing - O(n)
const int mod1 = 1e9 + 7;
ll add1(ll a, ll b) {
return (a + b) % mod1;
}
ll sub1(ll a, ll b) {
return (a - b + mod1) % mod1;
}
ll mul1(ll a, ll b) {
return (a * b) % mod1;
}
ll power1(ll a, ll b) {
if(!b)return 1;
ll pola = power1(a, b / 2);
pola = mul1(pola, pola);
if(b % 2)pola = mul1(pola, a);
return pola;
}
ll inv1(ll a) {
return power1(a, mod1 - 2);
}
// --------------------------------------------------------------
const int mod2 = 1e9 + 9;
ll add2(ll a, ll b) {
return (a + b) % mod2;
}
ll sub2(ll a, ll b) {
return (a - b + mod2) % mod2;
}
ll mul2(ll a, ll b) {
return (a * b) % mod2;
}
ll power2(ll a, ll b) {
if(!b)return 1;
ll pola = power2(a, b / 2);
pola = mul2(pola, pola);
if(b % 2)pola = mul2(pola, a);
return pola;
}
ll inv2(ll a) {
return power2(a, mod1 - 2);
}
const int p1 = 31;
const int p2 = 33;
ll pw1[maxn];
ll pw2[maxn];
ll invz1[maxn];
ll invz2[maxn];
void init_hsh() {
pw1[0] = 1;
pw2[0] = 1;
ff(i,1,maxn - 1) {
pw1[i] = mul1(pw1[i - 1], p1);
pw2[i] = mul2(pw2[i - 1], p2);
}
invz1[maxn - 1] = inv1(pw1[maxn - 1]);
invz2[maxn - 1] = inv2(pw2[maxn - 1]);
fb(i,maxn - 2, 0) {
invz1[i] = mul1(invz1[i + 1], p1);
invz2[i] = mul2(invz2[i + 1], p2);
}
}
ll hsh1[maxn];
ll get1(int l, int r) {
return mul1(sub1(hsh1[r], hsh1[l - 1]), invz1[l - 1]);
}
ll hsh2[maxn];
ll get2(int l, int r) {
return mul2(sub2(hsh2[r], hsh2[l - 1]), invz2[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) {
hsh1[i + 1] = add1(hsh1[i], mul1(pw1[i], s[i] - 'A' + 1));
hsh2[i + 1] = add1(hsh2[i], mul1(pw2[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 a11 = hsh1[i - 1];
ll a12 = mul1(pw1[i - 1], get1(i + 1, len + 1));
ll a1 = a11 + a12;
ll a21 = hsh2[i - 1];
ll a22 = mul2(pw2[i - 1], get2(i + 1, len + 1));
ll a2 = a21 + a22;
ll b1 = get1(len + 2, n);
ll b2 = get2(len + 2, n);
if(a1 == b1 && a2 == b2) {
rez += 1;
L = len + 2;
R = n;
}
}
ff(i,len + 1,n){
// brisem i-ti
ll a11 = get1(len + 1, i - 1);
ll a12 = mul1(pw1[i - len - 1], get1(i + 1, n));
ll a1 = a11 + a12;
ll a21 = get2(len + 1, i - 1);
ll a22 = mul2(pw2[i - len - 1], get2(i + 1, n));
ll a2 = a21 + a22;
ll b1 = hsh1[len];
ll b2 = hsh2[len];
if(a1 == b1 && a2 == b2){
rez += 1;
L = 1;
R = len;
}
}
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
**/
