#include<iostream>
#include<vector>
#include<algorithm>
#include<string>
#include<cmath>
#include<set>
#include<map>
#include<queue>
#include<stack>
#include<unordered_map>
#include<unordered_set>
using namespace std;
using ll = long long;
ll mod = 1e9 + 7;
ll gcd(ll a, ll b) {
if (b == 0)return a;
else return gcd(b, a % b);
}
bool check(string s, string t) {
int i = 0, j = 0;
while (i < s.size() && j < t.size()) {
if (s[i] == t[j]) {
j++;
}
i++;
}
if (j == t.size())return 1;
else return 0;
}
void solve() {
int n; cin >> n;
string s; cin >> s;
if (n % 2 == 0) {
cout << "NOT POSSIBLE" << endl;
return;
}
string a = s.substr(0, n/2),b=s.substr(0,n/2+1),c=s.substr(n/2,(n/2+1)),d = s.substr(n / 2+1, (n / 2));
if (check(b, d) && check(c, a)) {
if (d == a) {
cout << d << endl;
}
else cout << "NOT UNIQUE" << endl;
}
else if (check(b, d)) {
cout << d << endl;
}
else if (check(c, a)) {
cout << a << endl;
}
else {
cout << "NOT POSSIBLE" << endl;
}
}
signed main() {
ios_base::sync_with_stdio(0);
cin.tie(nullptr);
cout.tie(nullptr);
ll t = 1;
//cin >> t;
while (t--) {
solve();
}
}
