#include <bits/stdc++.h>
using namespace std;
using ll = long long;
const ll MOD1 = 1000000007;
const ll MOD2 = 1000000009;
const ll BASE = 91138233;
int main() {
ios::sync_with_stdio(false);
cin.tie(nullptr);
int N;
cin >> N;
string U;
cin >> U;
if (N % 2 == 0) {
cout << "NOT POSSIBLE\n";
return 0;
}
int n = (N - 1) / 2;
// Precompute powers
vector<ll> pow1(N + 1), pow2(N + 1);
pow1[0] = pow2[0] = 1;
for (int i = 1; i <= N; i++) {
pow1[i] = (pow1[i - 1] * BASE) % MOD1;
pow2[i] = (pow2[i - 1] * BASE) % MOD2;
}
// Prefix hashes
vector<ll> h1(N + 1, 0), h2(N + 1, 0);
for (int i = 0; i < N; i++) {
h1[i + 1] = (h1[i] * BASE + U[i]) % MOD1;
h2[i + 1] = (h2[i] * BASE + U[i]) % MOD2;
}
auto get_hash = [&](int l, int r) {
// [l, r)
ll x1 = (h1[r] - h1[l] * pow1[r - l]) % MOD1;
ll x2 = (h2[r] - h2[l] * pow2[r - l]) % MOD2;
if (x1 < 0) x1 += MOD1;
if (x2 < 0) x2 += MOD2;
return pair<ll,ll>{x1, x2};
};
set<string> candidates;
for (int i = 0; i < N; i++) {
// Build hash of V (U without i-th char)
// First half: length n
// Second half: length n
// We simulate indexing in V
// Map V index → U index
auto getV = [&](int l, int r) {
// returns hash of V[l, r)
// map to U skipping i
if (r <= i) {
return get_hash(l, r);
} else if (l >= i) {
return get_hash(l + 1, r + 1);
} else {
// split
auto left = get_hash(l, i);
auto right = get_hash(i + 1, r + 1);
int lenR = r - i;
ll x1 = (left.first * pow1[lenR] + right.first) % MOD1;
ll x2 = (left.second * pow2[lenR] + right.second) % MOD2;
return pair<ll,ll>{x1, x2};
}
};
auto hA = getV(0, n);
auto hB = getV(n, 2 * n);
if (hA == hB) {
// construct S explicitly (only when needed)
string S;
S.reserve(n);
for (int j = 0; j < N; j++) {
if (j == i) continue;
if ((int)S.size() < n)
S.push_back(U[j]);
}
candidates.insert(S);
if (candidates.size() > 1) {
cout << "NOT UNIQUE\n";
return 0;
}
}
}
if (candidates.empty()) {
cout << "NOT POSSIBLE\n";
} else {
cout << *candidates.begin() << "\n";
}
return 0;
}
