#include <iostream>
#include <vector>
using namespace std;
const int N = 2e6+3;
const int p1=31;
const int p2 = 101;
int n;
vector<bool> vis1(26);
vector<bool> vis2(26);
string s;
const int mod = 1e9 + 7;
int main()
{
cin >> n;
cin >> s;
if (n % 2 == 1) {
string w = "";
string e = "";
for (int i = 0;i<n/2;++i) {
w += s[i];
}
for (int i = n/2+1;i <n;++i) {
e += s[i];
}
string r = w + w;
string l = e+e;
bool fl1 = 0;
bool fl2 = 0;
int vl1 = 0;
int vl2 = 0;
for (int i = 0;i < n;++i) {
if (r[vl1]==s[i]) {
vl1++;
}
}
if (vl1 == n-1) {
fl1 = 1;
}
for (int i = 0;i < n;++i) {
if (l[vl2] == s[i]) {
vl2++;
}
}
if (vl2 == n - 1) {
fl2 = 1;
}
if (fl1 == 0 && fl2 == 0) {
cout << "NOT POSSIBLE";
}
else if (fl1 == 1 && fl2 == 1) {
if (w == e) {
cout << w;
}
else {
cout << "NOT UNIQUE";
}
}
else {
if (fl1 == 1) {
cout << w;
}
else {
cout << e;
}
}
}
else {
cout << "NOT POSSIBLE";
}
return 0;
}
