#include <bits/stdc++.h>
using namespace std;
#ifdef LOCAL
#include "debug.h"
#else
#define dbg(...) 47
#endif
#define int long long
constexpr int h = 41;
constexpr int mod = 1e9 + 7;
signed main() {
ios::sync_with_stdio(false);
cin.tie(nullptr);
int n;
string u;
cin >> n >> u;
if (n % 2 == 0) {
cout << "NOT POSSIBLE\n";
return 0;
}
vector<int> p(n + 1), pw(n + 1);
pw[0] = 1;
for (int i = 0; i < n; i++) {
pw[i + 1] = (pw[i] * h) % mod;
p[i + 1] = (p[i] * h + (u[i] - 'a' + 1)) % mod;
}
auto get = [&](int l, int r) {
if (r < l) return 0LL;
l++; r++;
return (p[r] - p[l - 1] * pw[r - l + 1] % mod + mod) % mod;
};
int len = n / 2;
string s;
int pos = -1;
for (int i = 0; i < n; i++) {
if (i <= len) {
int x = (get(0, i - 1) * pw[len - i] + get(i + 1, len)) % mod;
int y = get(n - len, n - 1);
if (x == y) {
string cur = u.substr(n - len, len);
if (!s.empty() && cur != s) {
cout << "NOT UNIQUE\n";
return 0;
}
s = cur;
}
} else {
int x = get(0, len - 1);
int y = (get(len, i - 1) * pw[n - i - 1] + get(i + 1, n - 1)) % mod;
if (x == y) {
string cur = u.substr(0, len);
if (!s.empty() && cur != s) {
cout << "NOT UNIQUE\n";
return 0;
}
s = cur;
}
}
}
if (s.empty()) {
cout << "NOT POSSIBLE\n";
} else {
cout << s << '\n';
}
return 0;
}
