#include <iostream>
#include <vector>
#include <map>
#include <unordered_map>
#include <unordered_set>
#include <set>
#include <queue>
#include <stack>
#include <cmath>
#include <string>
#include <stdio.h>
#include <fstream>
#include <climits>
#include <algorithm>
using namespace std;
class SegmentTree {
vector<int> tree;
int n;
void buildT(vector<int>& arr, int node, int start, int end) {
if (start == end) {
tree[node] = arr[start];
}
else {
int mid = (start + end) / 2;
buildT(arr, 2 * node + 1, start, mid);
buildT(arr, 2 * node + 2, mid + 1, end);
tree[node] = tree[2 * node + 1] + tree[2 * node + 2];
}
}
void updateT(int node, int start, int end, int idx, int value) {
if (start == end) {
tree[node] = value;
}
else {
int mid = (start + end) / 2;
if (idx <= mid)
updateT(2 * node + 1, start, mid, idx, value);
else
updateT(2 * node + 2, mid + 1, end, idx, value);
tree[node] = tree[2 * node + 1] + tree[2 * node + 2];
}
}
int queryT(int node, int start, int end, int l, int r) {
if (r < start || end < l)
return 0;
if (l <= start && end <= r)
return tree[node];
int mid = (start + end) / 2;
int leftQuery = queryT(2 * node + 1, start, mid, l, r);
int rightQuery = queryT(2 * node + 2, mid + 1, end, l, r);
return leftQuery + rightQuery;
}
void build(vector<int>& arr) {
n = arr.size();
tree = vector<int>(4 * n, 0);
buildT(arr, 0, 0, n - 1);
}
void update(int idx, int value) {
updateT(0, 0, n - 1, idx, value);
}
int query(int l, int r) {
return queryT(0, 0, n - 1, l, r);
}
};
long long int prime = 83, mod = 1e9 + 7;
int main()
{
ios_base::sync_with_stdio(false);
cin.tie(NULL);
/*freopen("paintbarn.in", "r", stdin);
freopen("paintbarn.out", "w", stdout);*/
int n;
cin>>n;
int answer = 0;
string ans = "";
string str;
cin >> str;
if (n % 2 == 1)
{
int ln = (n - 1) / 2;
string possibleAns = "";
int possible = 0;
possible = 1;
for (int i = 1; i <= ln; i++)
{
if (str[i] == str[i + ln])
{
possibleAns += str[i];
}
else
{
possible = 0;
possibleAns = "";
break;
}
}
if (possible)
{
if (ans != possibleAns)
answer++;
ans = possibleAns;
}
possibleAns = "";
possible = 0;
possible = 1;
for (int i = str.size() - 2; i >= str.size() - 1 - ln ; i--)
{
if (str[i] == str[i - ln])
{
possibleAns += str[i];
}
else
{
possible = 0;
possibleAns = "";
break;
}
}
if (possible)
{
reverse(possibleAns.begin(), possibleAns.end());
if (ans != possibleAns)
answer++;
ans = possibleAns;
}
possibleAns = "";
possible = 0;
string TheStart = "", TheEnd = "", saveEnd, saveStart;
long long int hashS = 0, hashE = 0, hashAll = 0;
for (int i = 0; i < ln; i++)
{
TheStart += str[i];
}
saveStart = TheStart;
TheStart = TheStart + TheStart;
for (int i = str.size() - ln; i <= str.size() - 1; i++)
{
TheEnd += str[i];
}
saveEnd = TheEnd;
TheEnd = TheEnd + TheEnd;
int d = 0;
vector <long long> prefS, prefE, pref, sufS(str.size() - 1), sufE(str.size() - 1), suf(str.size());
for (int i = 0; i < TheStart.size(); i++)
{
hashS = (hashS * prime + TheStart[i] - 'A') % mod;
hashE = (hashE * prime + TheEnd[i] - 'A') % mod;
prefS.push_back(hashS);
prefE.push_back(hashE);
}
for (int i = 0; i < str.size(); i++)
{
hashAll = (hashAll * prime + str[i] - 'A') % mod;
pref.push_back(hashAll);
}
hashS = 0;
hashE = 0;
hashAll = 0;
for (int i = TheStart.size() - 1; i >= 0; i--)
{
hashS = (hashS * prime + TheStart[i] - 'A') % mod;
hashE = (hashE * prime + TheEnd[i] - 'A') % mod;
sufS[i] = hashS;
sufE[i] = hashE;
}
for (int i = str.size() - 1; i >= 0; i--)
{
hashAll = (hashAll * prime + str[i] - 'A') % mod;
suf[i] = hashAll;
}
hashS = 0;
hashE = 0;
hashAll = 0;
for (int i = 1; i < str.size() - 1; i++)
{
long long int lHash = 0, rHash = 0;
lHash = pref[i - 1];
rHash = suf[i + 1];
if (i < ln)
{
long long int calculatedLeftHash, calculatedRightHash;
calculatedLeftHash = prefE[i - 1];
calculatedRightHash = sufE[i];
if (calculatedLeftHash == lHash && calculatedRightHash == rHash)
{
if (ans != saveEnd)
answer++;
ans = saveEnd;
}
}
else
{
long long int calculatedLeftHash, calculatedRightHash;
calculatedLeftHash = prefS[i - 1];
calculatedRightHash = sufS[i];
if (calculatedLeftHash == lHash && calculatedRightHash == rHash)
{
if (ans != saveStart)
answer++;
ans = saveStart;
}
}
}
}
if (answer == 0)
{
cout << "NOT POSSIBLE" << endl;
}
else if (answer > 1)
{
cout << "NOT UNIQUE" << endl;
}
else
{
cout << ans << endl;
}
}
