#include<bits/stdc++.h>
using namespace std;
string test(string a, string b) {
string f;
int i=0, j=0;
while (i<a.size() && i+j<b.size()) {
if (a[i]!=b[i+j]) j++;
else { f.push_back(a[i++]); }
} return f;
}
int main() {
cin.tie(0)->sync_with_stdio(false);
int n;
string s;
cin >> n >> s;
string first=test(s.substr(0, n/2), s.substr(n/2));
string second=test(s.substr(n/2+1), s.substr(0, n/2+1));
if (first.length()==n/2&&second.length()==n/2) cout << "NOT UNIQUE";
else if (first.length()==n/2) cout << first;
else if (second.length()==n/2) cout << second;
else cout << "NOT POSSIBLE";
return 0;
}
