#include <bits/stdc++.h>
using namespace std;
int n;
bool works(const string& s) {
int fails = 0;
for (int i = 0, j = n / 2; i < n / 2; i++, j++) {
if (s[i] != s[j]){
fails++;
if (fails > 1) return false;
j++;
}
}
return true;
}
int main() {
ios::sync_with_stdio(0);
cin.tie(0);
string u;
cin >> n >> u;
string s = "";
if (works(u))
s = string(u.begin(), u.begin() + n / 2);
string t = "";
for (int i = n / 2 + 1; i < n; i++)
t += u[i];
for (int i = 0; i <= n / 2; i++)
t += u[i];
if (works(t)) {
string v(t.begin(), t.begin() + n / 2);
if (s != "" && s != v) {
cout << "NOT UNIQUE\n";
return 0;
}
s = v;
}
if (s == "") cout << "NOT POSSIBLE\n";
else cout << s << '\n';
}
