#include <bits/stdc++.h>
using namespace std;
using i64 = long long;
using u64 = unsigned long long;
using ld = long double;
using ii = pair<int, int>;
bool subseq(string& a, string& b) {
int n = size(a);
vector<bool> pre(n + 1);
bool suf = true;
pre[0] = true;
for(int i = 1; i <= n; ++i) pre[i] = pre[i - 1] && a[i - 1] == b[i - 1];
if(pre.back()) return true;
for(int i = n - 1; i >= 0; --i) {
suf = suf && a[i] == b[i + 1];
if(pre[i] && suf) return true;
}
return suf;
}
void solve() {
string s;
int n;
cin >> n >> s;
if(~n & 1) {
cout << "NOT POSSIBLE\n";
return;
}
string a = s.substr(0, n / 2) + s.substr(0, n / 2);
string b = s.substr(n / 2 + 1) + s.substr(n / 2 + 1);
if(subseq(a, s)) {
if(a == b || !subseq(b, s)) cout << s.substr(0, n / 2) << '\n';
else cout << "NOT UNIQUE\n";
} else if(subseq(b, s)) cout << s.substr(n / 2 + 1) << '\n';
else cout << "NOT POSSIBLE\n";
}
int main() {
ios_base :: sync_with_stdio(false);
cin.tie(0);
int t = 1;
// cin >> t;
while(t--) solve();
return 0;
}
