#include <iostream>
#include <cmath>
#include <unordered_map>
#include <map>
#include <set>
#include <queue>
#include <vector>
#include <string>
#include <iomanip>
#include <algorithm>
#include <deque>
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
using namespace std;
typedef long long ll;
ll linf = 1e15+1;
inline void scoobydoobydoo(){
ios::sync_with_stdio(false);
ios_base::sync_with_stdio(false);
cin.tie(NULL); cout.tie(NULL);
}
const int MAXN = 2e6+1;
bool mem[MAXN][2];
int pref[MAXN][2];
int parent[MAXN];
int sill[MAXN];
int n;
bool foundFromRight;
int whatLetter[MAXN][30];
int letterPref[MAXN][30];
int rootOf(int x){
return (parent[x] == x ? x : parent[x] = rootOf(parent[x]));
}
void merge(int a, int b){
a = rootOf(a);
b = rootOf(b);
if (sill[a] < sill[b])swap(a, b);
parent[b] = a;
sill[a] += sill[b];
}
int main(){
scoobydoobydoo();
cin >> n;
string s; cin >> s;
if (n%2 == 0){
cout << "NOT POSSIBLE" << endl;
return 0;
}
int currentChecking = 0;
sill[0] = 1;
parent[0] = 0;
for (int x = 1; x < n; x++){
parent[x] = x;
sill[x] = 1;
if (s[x-1] == s[x]){
merge(currentChecking, x);
} else {
currentChecking = x;
}
}
int jS = (n-1)/2;
for (int i = 0; i <= n/2; i++){
if (i + jS > n)break;
if (s[i+jS] == s[i]){
mem[i][0] = true;
}
if (s[i+jS+1] == s[i]){
mem[i][1] = true;
}
}
pref[0][0] = mem[0][0];
pref[0][1] = mem[0][1];
for (int i = 1; i < n; i++){
pref[i][0] = pref[i-1][0] + mem[i][0];
pref[i][1] = pref[i-1][1] + mem[i][1];
}
/*
for (int i = 0; i < n; i++){
cout << "(" << pref[i][0] << " " << pref[i][1] << ") ";
}
cout << endl;
*/
int works = -1;
for (int i = 0; i <= n/2; i++){
//is this One
//HÖGRA + VÄNSTRA MÅSTE VARA LIKA MED jS
int leftJ = (i == 0 ? 0 : pref[i-1][1]);
int rightJ = pref[n/2][0]-pref[i][0];
if (leftJ+rightJ == jS){
if (works != -1 && rootOf(works) != rootOf(i)){
cout << "NOT UNIQUE" << endl;
return 0;
} else {
foundFromRight = false;
works = i;
}
}
}
for (int i = 0; i < n; i++){
pref[i][0] = 0;
pref[i][1] = 0;
mem[i][0] = false;
mem[i][1] = false;
}
for (int i = n/2; i < n; i++){
if (s[i-jS] == s[i]){
mem[i][0] = true;
}
if (s[i-jS-1] == s[i]){
mem[i][1] = true;
}
}
pref[n-1][0] = mem[n-1][0];
pref[n-1][1] = mem[n-1][1];
for (int i = n-2; i >= 0; i--){
pref[i][0] = pref[i+1][0] + mem[i][0];
pref[i][1] = pref[i+1][1] + mem[i][1];
}
/*
for (int i = 0; i < n; i++){
cout << "(" << pref[i][0] << " " << pref[i][1] << ") ";
}
cout << endl;
*/
for (int i = n-1; i >= n/2; i--){
//is this One
//DE TILL HÖGER MÅSTE HOPPA LÄNGRE, TILL VÄNSTER EJ
//HÖGER BORDE HA EN HOPP-PREFIX
int rightJ = (i == n-1 ? 0 : pref[i+1][1]);
int leftJ = pref[n/2][0]-pref[i][0];
if (leftJ+rightJ == jS){
if (works != -1 && works != i && rootOf(works) != rootOf(i)){
//cout << i << endl;
cout << "NOT UNIQUE" << endl;
return 0;
} else {
foundFromRight = true;
works = i;
}
}
}
if (works == -1){
cout << "NOT POSSIBLE" << endl;
return 0;
}
string ans = "";
if (foundFromRight){
for (int i = n/2; i < works; i++){
ans += s[i];
}
for (int i = works+1; i < n; i++){
ans += s[i];
}
cout << ans << endl;
return 0;
}
for (int i = 0; i < works; i++){
ans += s[i];
}
for (int i = works+1; i <= n/2; i++){
ans += s[i];
}
cout << ans << endl;
return 0;
}
