#include <bits/stdc++.h>
using namespace std;
const int MAXN = 2e5+1;
int N;
string inp;
// [i, j), [l, r)
bool oka(int i, int j, int l, int r) {
while(i < j) {
while(l < r && inp[i] != inp[l]) l++;
if(l >= r) break;
i++; l++;
}
return i == j;
}
string solve() {
const string notPossible = "NOT POSSIBLE";
const string notUnique = "NOT UNIQUE";
if(N % 2 == 0) return notPossible;
int m = N / 2;
bool okA = oka(0, m, m, N);
bool okB = oka(m+1, N, 0, m+1);
if(!okA && !okB) return notPossible;
if(okA && okB && inp.substr(0, m) != inp.substr(m+1, m)) return notUnique;
if(okA) return inp.substr(0, m);
return inp.substr(m+1, m);
}
int main() {
cin >> N >> inp;
cout << solve() << endl;
}
