#include <bits/stdc++.h>
using namespace std;
int main() {
cin.tie(nullptr)->sync_with_stdio(false);
#ifdef palilo
freopen("in", "r", stdin);
freopen("out", "w", stdout);
#endif
int n;
cin >> n;
if (!(n & 1)) return cout << "NOT POSSIBLE", 0;
string s;
cin >> s;
auto ok = [&](int i, int ed, int j) -> bool {
for (bool inserted = false; i != ed; ++i, ++j)
if (s[i] != s[j]) {
if (inserted) return false;
inserted = true;
--i;
continue;
}
return true;
};
bool front = ok(0, n >> 1, n >> 1), back = ok((n >> 1) + 1, n, 0);
if (front && back)
cout << (equal(s.begin(), s.begin() + (n >> 1), s.begin() + (n >> 1) + 1) ? s.substr(0, n >> 1) : "NOT UNIQUE");
else if (front)
cout << s.substr(0, n >> 1);
else if (back)
cout << s.substr((n >> 1) + 1);
else
cout << "NOT POSSIBLE";
}
