#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define sz size()
#define ff first
#define ss second
#define pb push_back
#define pii pair <int, int>
#define dur exit(0)
#define dur1 return(0)
const int N = 2e5 + 5;
int main () {
// freopen ("input.txt", "r", stdin);
ios::sync_with_stdio(0);cin.tie(0);cout.tie(0);
int n;
string s;
cin >> n >> s;
if (n % 2 == 0) {
cout << "NOT POSSIBLE\n";
return 0;
}
s = " " + s;
int san = 0, idx = 0;
for (int i = 1; i <= n; ++i) {
int ok = 0;
for (int j = 1; j <= n; ++j) {
if (j == i) continue;
int l = j, r = j + (n / 2);
if (l <= i and i <= r) {
r++;
}
if (r > n) break;
if (s[l] != s[r]) {
ok = 1;
break;
}
}
if (!ok) {
san++;
idx = i;
}
}
if (!san) {
cout << "NOT POSSIBLE\n";
}
else if (san == 1) {
int git = n / 2;
assert (idx != 0);
if (idx <= git) {
git++;
}
for (int i = 1; i <= git; ++i) {
if (idx == i) continue;
cout << s[i];
}
cout << "\n";
}
else {
cout << "NOT UNIQUE\n";
}
}
