// Success consists of going from failure to failure without loss of enthusiasm
#include <bits/stdc++.h>
using namespace std;
#define nl '\n'
#define pb push_back
#define pf push_front
#define mp make_pair
#define f first
#define s second
#define sz(x) int(x.size())
template<class T> using V = vector<T>;
using pi = pair<int, int>;
using vi = V<int>;
using vpi = V<pi>;
using ll = long long;
using pl = pair<ll, ll>;
using vpl = V<pl>;
using vl = V<ll>;
using db = long double;
template<class T> using pq = priority_queue<T, V<T>, greater<T>>;
const int MOD = 1e9 + 7;
const ll INFL = ll(1e17);
const int LG = 18;
int main() {
cin.tie(0)->sync_with_stdio(0);
int N; cin >> N;
string S; cin >> S;
if (N % 2 == 0) {
cout << "NOT POSSIBLE" << nl;
exit(0-0);
}
int M = (N - 1) / 2;
auto tri = [&](string T) {
int j = 0;
for(int i = 0; i < N; i++) {
if (j < 2*M && S[i] == T[j]) j++;
}
return j == 2 * M;
};
string A = S.substr(0, M);
string B = S.substr(N - M, M);
if (A == B) B.back() = '|';
bool a = tri(A + A), b = tri(B + B);
if (a && b) cout << "NOT UNIQUE" << nl;
else if (a) cout << A << nl;
else if (b) cout << B << nl;
else cout << "NOT POSSIBLE" << nl;
exit(0-0);
}
