#include <bits/stdc++.h>
#define lsb(x) (x & (-x))
#define ll long long
#define ull unsigned long long
// 217
// 44
using namespace std;
const int MAXN = (int) 2e6;
const int B = 37;
char str[MAXN + 1];
ull hsh[MAXN + 1], pw[MAXN + 1];
inline ull get(int l, int r) {
if(l > r)
return 0;
return hsh[r] - hsh[l - 1] * pw[r - l + 1];
}
inline void update(int st, int dr, int &l, int &r, ull &val) {
if(l == 0) {
l = st, r = dr, val = get(st, dr);
return ;
}
ull cur = get(st, dr);
if(cur != val) {
cout << "NOT UNIQUE";
exit(0);
}
}
int main() {
//ifstream cin("A.in");
//ofstream cout("A.out");
int i, n;
ios::sync_with_stdio(false);
cin.tie(0), cout.tie(0);
cin >> n >> str + 1;
if(n % 2 == 0) {
cout << "NOT POSSIBLE";
return 0;
}
pw[0] = 1;
for(i = 1; i <= n; i++) {
hsh[i] = hsh[i - 1] * B + str[i] - 'A' + 1;
pw[i] = pw[i - 1] * B;
}
int len = (n - 1) / 2;
int l = 0, r = 0;
ull val;
if(get(2, len + 1) == get(n - len + 1, n)) {
update(2, len + 1, l, r, val);
}
if(get(n - len, n - 1) == get(1, len)) {
update(n - len, n - 1, l, r, val);
}
for(i = 2; i < n; i++) {
ull a, b;
if(i <= len + 1) {
a = get(1, i - 1) * pw[len - i + 1] + get(i + 1, len + 1);
b = get(len + 2, n);
}
else {
a = get(1, len);
b = get(i + 1, n) + get(n - len, i - 1) * pw[n - i];
}
if(a == b) {
if(i <= len + 1) {
update(len + 2, n, l, r, val);
}
else {
update(1, len, l, r, val);
}
}
}
if(l == 0) {
cout << "NOT POSSIBLE";
return 0;
}
for(i = l; i <= r; i++) {
cout << str[i];
}
//cin.close();
//cout.close();
return 0;
}
