#include <bits/stdc++.h>
using namespace std;
using ll = long long;
const int maxn = 1e6+6;
const int M = 1e9+7;
const int base = 31;
int mult(int a, int b) {
return (1LL*a*b) % M;
}
int madd(int a, int b) {
return (a+b) % M;
}
int modexp(int a, int b) {
int res = 1;
while (b) {
if (b&1) res = mult(res,a);
a = mult(a,a);
b >>= 1;
}
return res;
}
int ctoi(char c) {
return c-'A'+1;
}
int N;
string U;
int pwr[maxn], ipwr[maxn];
int pre[maxn];
int sub(int l, int r) {
return mult((pre[r]-pre[l-1]+M)%M,ipwr[l-1]);
}
int comb(int l1, int r1, int l2, int r2) {
int h1 = sub(l1,r1);
int h2 = sub(l2,r2);
int shift = r1-l1+1;
return madd(h1,mult(h2,pwr[shift]));
}
map<int,pair<int,int>> ans;
int main()
{
ios_base::sync_with_stdio(false); cin.tie(NULL);
cin >> N >> U;
if (N % 2 == 0) {
cout << "NOT POSSIBLE\n";
return 0;
}
U = '#' + U;
pwr[0] = ipwr[0] = 1;
for (int i = 1; i <= N; i++) {
pwr[i] = mult(pwr[i-1],base);
ipwr[i] = modexp(pwr[i],M-2);
}
for (int i = 1; i <= N; i++) {
pre[i] = madd(pre[i-1],mult(ctoi(U[i]),pwr[i-1]));
}
vector<int> hashes; //hashes of the answers
int mid = N/2+1;
for (int i = 1; i <= N; i++) {
int h1, h2;
if (i <= mid) {
h2 = sub(mid+1,N);
if (i == 1) {
h1 = sub(2,mid);
}
else if (i == mid) {
h1 = sub(1,mid-1);
}
else {
h1 = comb(1,i-1,i+1,mid);
}
}
else {
h1 = sub(1,mid-1);
if (i == N) {
h2 = sub(mid,N-1);
}
else {
h2 = comb(mid,i-1,i+1,N);
}
}
//cout << i << ": " << h1 << ' ' << h2 << '\n';
if (h1 == h2) {
hashes.push_back(h1);
ans[h1] = (i <= mid) ? make_pair(mid+1,N) : make_pair(1,mid-1);
}
}
sort(hashes.begin(),hashes.end());
hashes.erase(unique(hashes.begin(),hashes.end()),hashes.end());
if (hashes.empty()) {
cout << "NOT POSSIBLE\n";
}
if (hashes.size() > 1) {
cout << "NOT UNIQUE\n";
}
else {
int l = ans[hashes[0]].first;
int r = ans[hashes[0]].second;
for (int i = l; i <= r; i++) {
cout << U[i];
}
cout << '\n';
}
}
