#include <iostream>
#include <string>
using namespace std;
string try_candidate(const string &s, int orig_start, int orig_len, int modified_start) {
bool was_mismatch = false;
int orig = orig_start, mod = modified_start, i = 0;
while (i < orig_len) {
if (s[orig] != s[mod]) {
if (was_mismatch)
return string();
was_mismatch = true;
mod++;
} else
orig++, mod++, i++;
}
return s.substr(orig_start, orig_len);
}
int main() {
string final;
int N;
cin >> N;
cin >> final;
int orig_len = (final.length() - 1) / 2;
const string&
pos1 = try_candidate(final, orig_len + 1, orig_len, 0),
pos2 = try_candidate(final, 0, orig_len, orig_len);
if ( (pos1.empty() && pos2.empty()) || (final.size() % 2 == 0))
cout << "NOT POSSIBLE" << endl;
else if (pos1.empty())
cout << pos2 << endl;
else if (pos2.empty())
cout << pos1 << endl;
else if (pos1 != pos2)
cout << "NOT UNIQUE" << endl;
else
cout << pos1 << endl;
return 0;
}
