/*
* Author: Nonoze
* Created: Tuesday 04/03/2025
*/
#include <bits/stdc++.h>
using namespace std;
#ifndef IN_LOCAL
#define dbg(...)
#endif
// #define cout cerr << "OUT: "
#define endl '\n'
#define endlfl '\n' << flush
#define quit(x) return (void)(cout << x << endl)
template<typename T> void read(T& x) { cin >> x;}
template<typename T1, typename T2> void read(pair<T1, T2>& p) { read(p.first), read(p.second);}
template<typename T> void read(vector<T>& v) { for (auto& x : v) read(x); }
template<typename T1, typename T2> void read(T1& x, T2& y) { read(x), read(y); }
template<typename T1, typename T2, typename T3> void read(T1& x, T2& y, T3& z) { read(x), read(y), read(z); }
template<typename T1, typename T2, typename T3, typename T4> void read(T1& x, T2& y, T3& z, T4& zz) { read(x), read(y), read(z), read(zz); }
template<typename T> void print(vector<T>& v) { for (auto& x : v) cout << x << ' '; cout << endl; }
#define sz(x) (int)(x.size())
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
#define make_unique(v) sort(all(v)), v.erase(unique(all(v)), (v).end())
#define pb push_back
#define mp(a, b) make_pair(a, b)
#define fi first
#define se second
#define cmin(a, b) a = min(a, b)
#define cmax(a, b) a = max(a, b)
#define YES cout << "YES" << endl
#define NO cout << "NO" << endl
#define QYES quit("YES")
#define QNO quit("NO")
#define int long long
#define double long double
const int inf = numeric_limits<int>::max() / 4;
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
const int MOD = 1e9+7, LOG=20;
void solve();
signed main() {
ios::sync_with_stdio(0);
cin.tie(0);
int tt=1;
// cin >> tt;
while(tt--) solve();
return 0;
}
bool works(string a, string b) {
int n=sz(a);
while (!a.empty() && b.back()==a.back()) b.pop_back(), a.pop_back();
if (!a.empty()) b.pop_back();
while (!a.empty() && b.back()==a.back()) b.pop_back(), a.pop_back();
return a.empty();
}
int n, k, m, q;
string s;
void solve() {
read(n, s);
if (n%2==0) quit("NOT POSSIBLE");
n/=2;
bool a=works(s.substr(0, n), s.substr(n, n+1));
bool b=works(s.substr(n+1, n), s.substr(0, n+1));
if (!a && !b) quit("NOT POSSIBLE");
if (a && b && s.substr(0, n)!=s.substr(n+1, n)) quit("NOT UNIQUE");
if (b) quit(s.substr(n+1, n));
quit(s.substr(0, n));
}
