#include <bits/stdc++.h>
#define all(v) v.begin(), v.end()
#define pb push_back
#define F first
#define S second
#define in insert
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
const int N = 1e6 + 123;
const int P = 320;
const int mod = 1e9 + 7;
ll n, m, k, q;
string s;
void salemkhan() {
cin >> n >> s;
if(n % 2 == 0) {
cout << "NOT POSSIBLE";
return;
}
string cur1 = "";
string cur2 = "";
for(int i = 0; i < n / 2; i++) {
cur1 += s[i];
}
for(int i = n / 2 + 1; i < n; i++) {
cur2 += s[i];
}
bool f1, f2;
f1 = f2 = 0;
ll l = 0;
for(int i = n / 2; i < n; i++) {
if(cur1[l] == s[i]) {
l++;
}
}
if(l == n / 2) {
f1 = 1;
}
l = 0;
for(int i = 0; i <= n / 2; i++) {
if(cur2[l] == s[i]) l++;
}
if(l == n / 2) {
f2 = 1;
}
if(f1 && f2) {
cout << "NOT UNIQUE";
}
else if(f1 && !f2) {
cout << cur1;
}
else if(f2 && !f1) {
cout << cur2;
}
else cout << "NOT POSSIBLE";
}
// 28 37 48 49 55 56 57 58 67 9
// 9 + 1 +
int main() {
//freopen("knight.in", "r", stdin);
//freopen("knight.out", "w", stdout);
ios_base::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
ll tt = 1;
//cin >> tt;
while(tt--) {
salemkhan();
}
}
